A  

acicular: the descriptive term for a leaf that is slender and pointed, needle-like. See illustration.

acidic: having the properties of an acid, or containing acid; having a pH below 7. Often contrasted with alkaline or basic.

acuminate: the descriptive term for a leaf that is tapering to a long point. See illustration.

acute: the descriptive term for a leaf that is pointed, having a short sharp apex angled less than 90º.

adventitious: arising from an organ other than root; usually lateral.

adventitious buds: dormant buds which form on stumps or roots after cutting or injury.

alga, pl. algae: a simple nonflowering plant of a large group that includes the seaweeds and many single-celled forms. Algae contain chlorophyll but lack true stems, roots, leaves, and vascular tissue.

alkaline: having the properties of an alkali, or containing alkali; having a pH greater than 7. Often contrasted with acid or acidic; same as basic.

allelopathy: is a biological phenomenon by which an organism produces one or more biochemicals that influence the growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial (positive allelopathy) or detrimental (negative allelopathy) effects on the target organisms. One example is the chemical juglone that is secreted by black walnuts and kills many other plants such as most rhododendrons and azaleas. Conversely, some rhododendrons produce phenols and other chemicals that will inhibit plants planted near them. That is one thing that is so insidious about R. ponticum in Britain. Allelopathy assists ponticum to run rampant and kill nearby plants and completely take over areas.

alternate: (of leaflets, leaves or shoots) placed alternately on the two sides of the stem. See illustration.

angiosperms: (meaning "covered seed") are flowering plants. They produce seeds enclosed in an ovary in fruit. They are the dominant type of plant today; there are over 250,000 species. Their flowers are used in reproduction. Angiosperms evolved about 145 million years ago, during the late Jurassic period, and were eaten by dinosaurs. They became the dominant land plants about 100 million years ago (edging out conifers, a type of gymnosperm). Angiosperms are divided into the monocots (like grasses and corn) and dicots (like beans).

anther: the part of a stamen that bears the pollen. Pollen sac.

apex: the tip of a leaf.

aphid: a minute bug that feeds by sucking sap from plants. It reproduces rapidly, often producing live young without mating.

apical: at the top, tip or end of a structure, terminal.

apical dominance: when the terminal bud inhibits growth of lateral vegetative buds.

apogamy: is when a seed is produced without the gametes fusing. The resulting plant usually resembles the seed parent and is sterile.

arborescent: tree-like in appearance and size.

arcuate: the descriptive term for a leaf with secondary veins bending toward the apex. See illustration.

aristate: the descriptive term for a leaf ending in a stiff, spine-like tip. See illustration.

asexual reproduction: reproduction by vegetative methods such as cuttings, layering, grafting, or tissue culture. Frequently called propagation. [See cuttings, layering, grafting, or tissue culture.]

asymmetrical: the descriptive term for a leaf with a blade shape different on each side of the midrib.

auxin: a plant hormone that causes the elongation of cells in shoots and is involved in regulating plant growth. It is commonly used to assist in the productions of roots on cuttings.

axil: the upper angle between a leaf stalk or branch and the stem or trunk from which it is growing.

axillary bud: a bud that grows from the axil of a leaf and may develop into a branch or flower cluster. Also called lateral bud.

azalea: a type of rhododendron. If flowers grow from terminal buds, new leaves and shoots grow from lateral buds and leaves are deciduous, then the rhododendron is an azalea in the Pentanthera subgenus. If flowers and leaves grow from the same terminal buds, and the flowers have 5 to 10 stamens, then the rhododendron is an azalea in the Tsutsusi subgenus. Typically an azalea is either evergreen with flowers individually rather than in a truss or is deciduous and may have flowers singly or in a ball-shaped truss. On the other hand, a rhododendron that is not an azalea is evergreen and has flowers that are in trusses (a truss is a flower-like structure composed of many flowers). (azalea is from Greek, feminine of azaleos ´dry,´ because the shrub flourishes in dry soil.)

azaleodendron: Azaleodendrons are hybrid crosses between elepidote rhododendrons and deciduous azaleas except the Hardijzer's which are crosses of the lepidote, R. racemosum, with evergreen azaleas. Hardijzer's crosses between the lepidote, R. racemosum, and evergreen azaleas have been lumped with the Azaleodendrons because there is no other subgenus created for them.

B

bacterium, pl. bacteria: members of a large group of unicellular microorganisms that have cell walls but lack organelles and an organized nucleus, including some that can cause disease. Bacteria are widely distributed in soil, water, and air, and on or in the tissues of plants and animals. Formerly included in the plant kingdom, they are now classified separately (as prokaryotes).

bark: tissues of plant that are outside wood or xylem.

bark split: occurs when sap is flowing from the roots through the xylem and a freeze occurs. This freezing causes the watery substance in the xylem to freeze and rupture the bark and cambium layer. See Bark split in the problems section.

basic: having the properties of a base, or containing a base; having a pH greater than 7. Often contrasted with acid or acidic; same as alkaline.

beetle: an insect of an order distinguished by forewings typically modified into hard wing cases (elytra) that cover and protect the hind wings and abdomen. Weevils are a type of beetle. A member of order Coleoptera, which is the largest order of animals on the earth.

bi-generic: hybrid with parents in two different genera.

bipinnate: the descriptive term for a leaf with each leaflet also pinnate.

blade: the descriptive term for the bladelike part of a leaf beyond the stalk or petiole.

blossom: a flower or a mass of flowers on a tree or bush.

bloom: a waxy coating found on stems, leaves, flowers and fruits, usually of a grayish cast and easily removed.

blotch: area of contrasting color on the uppermost petal(s) of the flowers of some plants.

botany: the scientific study of plants, including their physiology, structure, genetics, ecology, distribution, classification, and economic importance.

bract: a modified leaf or scale, typically small, with a flower or flower cluster in its axil. Bracts are sometimes larger and more brightly colored than the true flower, as in a poinsettia.

bristle: a slender stiff hair.

bud: a compact knoblike growth on a plant that develops into a leaf, flower, or shoot.

bud scale: leaflike structures surrounding a flower bud.

bug: A member of the order Hemiptera, a large order of insects that comprises the true bugs, which include aphids, cicadas, leafhoppers, and many others. They have piercing and sucking mouthparts and incomplete metamorphosis.

bullate: Blistered or puckered like the leaf of a primrose .

C

calcareous: limy or chalky rock or soil with a high proportion of calcium carbonate.

callus: healing growth over a wound or cut.

calyx: the sepals of a flower, typically forming a whorl that encloses the petals and forms a protective layer around a flower in bud. The lowermost whorl of modified leaves, sepals. The calyx can be so large as to give the appearance of a double flower.

cambium: the layer of actively dividing meristemic cells between the wood, and bark of stems or roots. The cambium forms the specialized xylem and phloem cells and causes the stem to increase in thickness. A green cambium layer is a sign of life in a woody stem.

campanulate: bell-shaped.

capsule: seedpod.

carpel: the female reproductive organ of a flower, consisting of an ovary, a stigma, and usually a style. Same as pistil.

caterpillar: the larva of a butterfly or moth, having a segmented wormlike body with three pairs of true legs and several pairs of leg-like appendages. Caterpillars may be hairy, have warning coloration, or be colored to resemble their surroundings. Caterpillars (moths and butterflies) are insects in the order Lepidoptera.

cell: the smallest structural and functional unit of an organism, typically microscopic and consisting of cytoplasm and a nucleus enclosed in a membrane.

cell division: the process by which a parent cell divides into two daughter cells which includes mitosis followed by cytokinesis called vegetative reproduction.

chlorophyll a green pigment, present in all green plants and in cyanobacteria, responsible for the absorption of light to provide energy for photosynthesis. Its molecule contains a magnesium atom.

chlorosis: leaves partly or wholly pale green or yellowed due to a lack of chlorophyll . It is usually caused by a nutrient problem caused by a troublesome soil pH or low nutrient concentration.

chromosome:: microscopic, self-replicating molecule found in the nucleus of cells. Chromosomes contain genetic material (a single DNA molecule that contains many genes). The genome of an organism is made up of the set of chromosomes that contain all of its genes. Chromosomes were discovered by Walther Fleming in 1879; the term chromosome was proposed by Waldeyer in 1888.

ciliate: with cilium, applied to leaves having fine hairs along the leaf margin. See illustration.

cilium, pl. cilia: small hairs, often present on leaf margins.

clasping: a type of leaf in which the blade partially or wholly surrounds the stem, often giving the impression that the shoot grows through the leaf..

class: a principal taxonomic grouping that ranks above order and below phylum or division

classification: the arrangement of animals and plants in taxonomic groups according to their observed similarities (including at least kingdom , phylum in zoology, division in botany, and class, order, family, genus, and species). See the section on Classification. The genus Rhododendron is part of the family Ericaceae, order Ericales, subclass Dilleniidae, class Magnoliopsida, division Magnoliophyta, superdivision Spermatophyta, subkingdom Tracheobionta, and kingdom Plantae.

clone: a group of organisms or cells produced asexually (by vegetative methods such as cuttings, layering, or tissue culture) from one ancestor or stock, to which they are genetically identical.

cold hardiness: clones of a plant are typically thought to be able to withstand a certain low temperature in winter without sustaining plant or bud damage. They system used to measure cold hardiness is the hardiness rating.

compound leaf: a leaf of a plant consisting of several or many distinct parts (leaflets) joined to a single stem. Not simple.

cordate: heart-shaped, applied to leaves having the petiole at the broader and notched end. See illustration.

corolla: the petals of a flower, typically forming a whorl within the sepals and enclosing the reproductive organs.

cotyledon: a seed leaf, not a true leaf, but one of the two embryonic leaf-like structures that emerge from the seeds of dicots such as rhododendrons and azaleas. Note: monocots are characterized by having only one cotyledon.

crawler: the mobile nymph stage of certain insects such as scale insects, and mealybugs. Later nymphs become somewhat or completely immobile.

crenate: applied to leaves having rounded teeth along the leaf margin. See illustration.

cross-venulate: the descriptive term for a leaf with small veins connecting the secondary veins. See illustration.

crown: the junction of the roots and the stem. Also called root collar.

cultivar: a cultivated variety, an asexually propagated, cultivated form of a plant. In rhododendrons, a hybrid cultivar is named in the format Rcv. 'Cynthia'. A species cultivar is named in the format R. arboreum 'Bhutan'.

cultivate: break up soil in preparation for sowing or planting or to prevent weeds from being established. Note, rhododendrons and azaleas have very shallow roots and cultivating can easily damage the roots.

cuneate: the descriptive term for a leaf that is wedge-shaped. See illustration.

cutting: new growth cut for rooting or other vegetative propagation.

cutting graft: a graft on to a cutting that will be rooted and grafted simultaneously.

cytoplasm: it is comprised of cytosol, a gel-like substance enclosed within the cell membrane, and organelles, the cell's internal sub-structures.

cytokinesis: the second phase of cell division after mitosis in the nuclei, cytoplasm, organelles, and cell membrane divides into two cells containing roughly equal shares of these cellular components.

cytokinin: a growth-promoting substance favoring shoot formation.

D

deadhead: remove spent flowers from a plant so that energy goes into growth instead of seed production..

deciduous: shedding its leaves annually. Often contrasted with evergreen .

deltoid: the descriptive term for a leaf that is triangular and the stem attaches to a side. See illustration.

dentate: applied to leaves having symmetrical pointed teeth along the leaf margin. See illustration.

denticulate: applied to leaves having fine symmetrical pointed teeth along the leaf margin. See illustration.

dichotomous: the descriptive term for a leaf with veins branching symmetrically in pairs. See illustration.

dicot: is a type of flowering plant (an angiosperm) whose seed has two embryonic leaves (cotyledons). The leaf veins are usually net-like (and not parallel).

dimorphic: having two distinct shapes or forms. Evergreen azaleas are dimorphic, having spring leaves that unfold at the beginning of the growing season and are dropped in autumn and summer leaves that emerge in early summer and are smaller, thicker, darker, and more leathery than spring leaves.

diploid: describing a cell that has two sets of chromosomes or organisms with such cells. Most organisms have cells that are diploid, except the cells composing the gametes, like the pollen grain and the egg, which are haploid. Cells with more than two sets of chromosomes are said to be polyploid.

discoid: shaped like a disc.

division: a principal taxonomic category in botany that ranks above class and below kingdom, equivalent to the phylum in zoology.

DNA: (short for deoxyribonucleic acid) is a complex organic molecule that carries the genetic information of an organism. DNA is the primary constituent of chromosomes. DNA is made up of two strands of amino acid bases (adenosine, cytosine, guanine, and thymine) arranged in a double helix. DNA's chemical instructions are carried throughout the cell by another nucleotide, RNA.

dormant: alive but not actively growing.

double: a flower type in which all the stamens have been transformed completely into petals, which may be similar in size and shape to the corolla petals, and the pistil may have been transformed, but the calyx has not been transformed.

double hose-in-hose: a flower type in which the stamens and calyx have all been transformed into petals.

doubly serrate: the descriptive term for a serrate leaf having little teeth on larger teeth. See illustration.

drained: make (land) drier by providing channels for water to flow away and percolate to the subsoil.

drought: a prolonged period of abnormally low rainfall.

E

egg: the female reproductive cell in animals and plants; an ovum.

elepidote: without scales, as opposed to lepidote (with scales). All azaleas and elepidote rhododendrons are elepidotes.

elliptic: the descriptive term for a leaf that is oval with a short point or no point. See illustration.

embryo: the part of a seed that develops into a plant, consisting (in the mature embryo of a higher plant) of a plumule, a radicle, and one or two cotyledons.

endemic: native or confined naturally to a given geographical area.

entire: the descriptive term for a leaf having an even smooth edge with neither teeth nor lobes. See illustration.

epiphytic: a plant growing non-parasitically upon another. Many Vireya Rhododendrons are epiphytic.

Ericaceae family: a botanical family. Members of the Ericaceae family include the fruit-bearing blueberries, cranberries and huckleberries as well as the rhododendron, azalea, arbutus, trailing arbutus (Mayflower), heather, mountain laurel (genus Kalmia) and true heaths (genus Erica ). The Ericaceae are often called heath family or the family of "acid-loving plants" since they tolerate acidic soils quite well, and many require an acidic soil. The family Ericaceae is part of the order Ericales, subclass Dilleniidae, class Magnoliopsida, division Magnoliophyta, superdivision Spermatophyta, subkingdom Tracheobionta, and kingdom Plantae.

ericaceous: belong to the family Ericaceae, which includes rhododendrons, heathers, mountain laurel, blueberries, etc.

evergreen: of or denoting a plant that retains green leaves throughout the year. Often contrasted with deciduous.

exserted: protruding.

F

falcate: the descriptive term for a leaf that is hooked or sickle shaped. See illustration.

family: a principal taxonomic grouping that ranks above genera and below order usually ending in -idae (in zoology) or -aceae (in botany).

farinaceous: powdery.

feral: wild or escaped from domestication and able to reproduce normally.

fertilize: the fusion of gametes to initiate the development of a new individual organism. On plants the fertilization process is complex. After the carpel is pollinated, the pollen grain germinates. From each pollen grain, a pollen tube grows out that attempts to travel to the ovary by creating a path through the female tissue. The vegetative and generative nuclei of the pollen grain pass into the pollen tube. After the pollen grain adheres to the stigma of the carpel a pollen tube grows and penetrates the ovule through a tiny pore called a micropyle. The pollen tube does not directly reach the ovary in a straight line. It travels near the skin of the style and curls to the bottom of the ovary, then near the receptacle, it breaks through the ovule through the micropyle (an opening in the ovule wall) and the pollen tube "bursts" into the embryo sac.

fertilizer: a chemical or natural substance added to soil or land to increase its fertility. Too much actually decreases fertility.

filament: the slender part of a stamen that supports the anther.

filiform: the descriptive term for a leaf that is thread or filament shaped.

flabellate: the descriptive term for a leaf that is semi-circular or fan-shaped. See illustration.

floret: grouping of small flowers as in family compositeae, but never to be used for a rhododendron flower.

flower: the seed-bearing part of a plant, consisting of reproductive organs (stamens and a pistil (carpel) that are typically surrounded by a brightly colored corolla (petals) and a green calyx (sepals). Flowers, especially azalea flowers, have many different forms: single, hose-in-hose, semi-double, semi-double hose-in-hose, double, double hose-in-hose, and spider.

foliage: plant leaves, collectively.

force: subject a dormant plant to warm temperatures and light to break dormancy and begin blooming and/or vegetative growth.

form, pl. formae: a taxonomic category that ranks below variety, which contains organisms differing from the typical kind in some trivial, frequently impermanent, character, e.g., a color variant. In cultivated plants, it is current practice to name these forms as cultivars.

fruit: the seed-bearing structure of a plant, e.g., an acorn or apple.

fungi , pl. fungus: any of a group of unicellular, multicellular, or syncytial spore-producing organisms feeding on organic matter, including molds, yeast, mushrooms, and toadstools. Fungi lack chlorophyll and are therefore incapable of photosynthesis. Many play an ecologically vital role in breaking down dead organic matter; and others cause disease. The familiar mushrooms and toadstools are merely the fruiting bodies of organisms that exist mainly as a threadlike mycelium in the soil. Some fungi form associations with other plants, growing with algae to form lichens, or in the roots of higher plants to form mycorrhizae. Fungi are now often classified as the separate kingdom, Fungi, which is distinct from Animalia (animals), Plantae (plants), Protista (protozoans and eucaryotic algae), and Monera (blue-green algae).

fungicides: chemicals that prevent or retard the growth of fungi.

G

gall: an abnormal growth formed on plants in response to the presence of insect larvae, mites, or fungi.

gamete: a the male or female reproductive cell of an organism (the pollen grain or the egg). Each gamete has only half the number of chromosomes that the other cells of that organism have (it is haploid).

gene: a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring through proteins coded directly by genes. Technically a gene is a distinct sequence in a portion of the DNA molecule that forms a chromosome.

genetic makeup: refers to the hereditary make up of a plant which is contained in all its cells in the genes, chromosomes, DNA and RNA.

genome: refers to the hereditary make up of a plant which is contained in all its cells in the genes, chromosomes, DNA and RNA.

genus pl. genera: a principal taxonomic category that ranks above species and below family, and is denoted by a capitalized Latin name, e.g., Rhododendron. Note: azaleas are members of the genus Rhododendron.

germinate: (of a seed or spore) begin to grow and put out shoots after a period of dormancy.

girdling: an path of destruction encircling a root or stem of a plant, resulting in interruption of the flow of water and/or nutrients.

glabrous: smooth, free of hair.

glacous: having a gray or whitish cast, covered with a bluish, gray, or white bloom.

glandular hair: hair bearing a knob-like secretory swelling (head) at its apex.

glaucous: covered with a waxy bloom or whitish material that rubs off readily.

globose: nearly spherical.

grex: all the seedlings of a particular cross. The various selections of the rhododendron hybrid 'PJM' are all from a grex.

H

hair: Plant hairs may be unicellular or multicellular, branched or unbranched. Multicellular hairs may have one or several layers of cells. Branched hairs can be dendritic (tree-like), tufted, or stellate (star-shaped). Plant hairs are a type of trichome. On rhododendrons, they are usually called indumentum (if on underside of a leaf) or tomentum (if on the upper surface of a leaf).

haploid: describing a cell that has only half the number of chromosomes that the other cells of that organism have (most are diploid). Gametes (like the pollen grain and the egg) are haploid.

hardiness rating: a system to classify the coldest temperature zone a plant has been exposed to in winter without sustaining plant or bud damage. The American System is: H1: -25 °F; H2: -15 °F; H3: -5 °F; H4: +5 °F; H5: +15 °F. The Royal Horticultural Society System is: H9: -30 °F; H8: -25 °F; H7: -20 °F; H6: -15 °F; H5: -10 °F; H4: -5 °F; H3: 0 °F; H2: 5 °F; H1: 10 °F; H0: 30 °F.

hardiness zone: a region in which the lowest expected temperature corresponds to a certain hardiness rating. Hardiness zones are updated periodically to compensate for climate change. USDA Hardiness Zones are as follows: 4A: -30 °F; 4B: -25 °F; 5A: -20 °F; 5B: -15 °F; 6A: -10 °F; 6B: -5 °F; 7A: 0 °F; 7B: 5 °F; 8A: 10 °F; 8B: 15 °F; 9A: 20 °F; 9B: 25 °F; 10A: 30 °F.

hardpan: is a general term for a dense layer of soil, residing usually below the uppermost topsoil layer. There are different types of hardpan, all sharing the general characteristic of being a distinct soil layer that is largely impervious to water.

hardy: (of a plant) able to survive outside during winter, not tender. Hence, what is hardy depends on the local climate.

hastate: the descriptive term for a leaf that is triangular with basal lobes, shaped like a spear point with flaring pointed lobes at the base. See illustration.

hexaploid: describing a cell that has six paired sets of chromosomes. Most organisms have cells that are diploid, i.e. having two sets of chromosomes. Also, describing a plant with hexaploid cells.

heath family: the heath family is principally composed of evergreen shrubs that grow in acidic and infertile soils. Since heaths thrive on acidic soil, they are often found on moors, swamps and mountainsides. See Ericaceae family.

herbaceous: plants with no cambium layer. A herbaceous plant has leaves and stems that die down at the end of the growing season to the soil level. They have no persistent woody stem above ground. Herbaceous plants may be annuals, biennials or perennials.

herbicide: a chemical that kills plants.

hirsute: with rough or course, more or less, erect hairs.

honeydew: the term for the excretory material of aphids and many other sucking pests.

hormone: a regulatory substance produced in an organism and transported in tissue fluids such as blood or sap to stimulate specific cells or tissues into action. See auxin.

hose-in-hose: a flower type in which the calyx is transformed into petals, such that it appears as two similar corollas, one inside the other and rotated so that all the petals are visible.

humus: the organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms.

hybrid: the offspring of two plants or animals of different species or varieties, such as a hybrid of wheat and rye. A cross between different varieties of the same species is an intraspecific hybrid.

hybridize: crossbreed individuals of two different species or varieties.

I

indumentum, pl. indumenta: in plants, a covering of hairs. On rhododendrons it is commonly a woolly covering on the underside of mature leaves, while loose hairs on the upper side of leaves would be called tomentum. Rhododendrons with indumentum typically do not have scales and are hence elepidotes. Some lepidotes, such as R. edgeworthii, also have indumentum which may obscure the scales leading them to be mistaken for elepidotes occasionally. Indumentum is a type of trichome. Also see plastered indumentum.

inflorescence: the complete flower head of a plant including stems, stalks, bracts, and flowers.

infructescence: the inflorescence of a plant after the bloom has finished and it is in the fruiting stage.

insect: a small arthropod animal that has six legs and generally one or two pairs of wings. Some types of insects that attack rhododendron and azaleas include caterpillars (moths), scale, beetles, bug, and thrip. Contrast with mites and nematodes that are not insects.

involute: rolled inward or toward the upper surface.

iron chelate: a soluble form of ferrous sulfate applied to the soil to prevent or overcome an iron deficiency.

ironclad: Some of the old R. catawbiense hybrids and others of similar breeding considered to be super winter-hardy. They are generally hardy to -25 °F. Included are: 'Album Elegans', 'Album Grandiflorum', 'America', 'Atrosanguineum', 'Boule de Neige', 'Caractacus', 'Catawbiense Album, 'Catawbiense Boursault', Charles Dickens, 'English Roseum', 'Everestianum', 'Henrietta Sargent', 'H.W. Sargent', 'Ignatious Sargent', 'Lady Armstrong', 'Lee's Dark Purple', 'Mrs. Charles S. Sargent', 'Nova Zembia', 'Purpureum Elegans', 'Purpureum Grandiflorum', 'Roseum Elegans', and 'Roseum Pink'.

J juvenile: an early phase of plant growth in which it increases in size but has not yet flowered.
K

kingdom: the highest grouping of similar organisms. A kingdom contains one or more division in botany or phylum in zoology. Life on Earth is divided into five kingdoms: Animalia (animals), Plantae (plants), Fungi, Protista (protozoans and eucaryotic algae), and Monera (blue-green algae).

L

laciniate: the descriptive term for a leaf that is very deeply lobed, the lobes being very drawn out.

lamina: the blade of a leaf.

laminar: the descriptive term for a leaf that is flat.

lanceolate: the descriptive term for a leaf that is long and wider in the middle and pointed at both ends. See illustration.

lateral bud: a bud that grows from the axil of a leaf and may develop into a branch or flower cluster. Also called axillary bud.

larva, pl. larvae: the active immature form of an insect, esp. one that differs greatly from the adult and forms the stage between egg and pupa, e.g., a caterpillar or grub.

lax truss: a truss in which the flowers hang down.

leaf, pl. leaves: a flattened structure of a higher plant, typically green and bladelike, that is attached to a stem directly (sessile) or via a stalk (petiole). Leaves are the main organs of photosynthesis and transpiration.

Attachment, Compound Leaves, Edges, Shapes, Venation, Repetition


Leaf Attachment Types
peltate
 
perfoliate
 
petiolated
 
sessile
     
back to Leaf Morphology Index

Compound Leaf Blade Types (Compound Leaves have multiple leaflets, a Simple Leaf has one leaflet)
pinnate-even
 
pinnate-odd
 
rosette
   
trifoliate/ternate
 
tripinnate
 
whorled
   
back to Leaf Morphology Index

Leaf Edge (Margin) Types
ciliate   crenate   dentate   denticulate   doublyserrate  
entire
         
lobate
 
serrate
 
serrulate
 
sinuate
 
spiny
  undulate
         
back to Leaf Morphology Index

Leaf Shape Types
acicular   acuminate   aristate   cordate   cuneate   deltoid   elliptic
           
falcate   flabellate   hastate   lanceolate   linear   lobed   obcordate  
             
obovate   obtuse   orbicular   ovate   palmate   pedate   pinnatisect
           
reniform   rhomboid   spatulate   spear-shaped   subulate   truncate   unifoliate
           
back to Leaf Morphology Index

Venation, Leaf Vein Types
arcuate
 
cross-venulate
 
dichotomous
 
longitudinal
 
palmate
 
parallel
 
pinnate
 
reticulate
 
rotate
               
back to Leaf Morphology Index

Leaf Repetition Types
alternate
 
opposite
 
whorled
   
back to Leaf Morphology Index

leaf miner: a small fly, moth, beetle, or sawfly whose larvae burrow between the two epidermal surfaces of a leaf.

leaflet: each of the leaflike structures that make up a compound leaf.

lepidote: having minute scales. Tiny scales typically cover the undersides of the leaves. Characteristic used to separate the genus Rhododendron into two major groups: lepidotes and elepidotes which have no scales. Some lepidotes, such as R. edgeworthii, also have indumentum which may obscure the scales leading them to be mistaken for elepidotes occasionally. The tropical vireya rhododendrons are lepidote rhododendrons. However, all azaleas are elepidotes.

linear: the descriptive term for a leaf that is long with parallel margins. See illustration.

lobate: the descriptive term for a leaf with indented margin but not indented to the midrib. See illustration.

lobe: usually applies to the outer parts of the corolla or calyx.

lobed: the descriptive term for a leaf with deeply indented margin. See illustration.

longitudinal: the descriptive term for a leaf with veins aligned mostly along the long axis of the leaf. See illustration.

M

margin: the edge of a leaf or flower.

mealybugs: small, sap-sucking scale insects that are coated with a white, powdery wax that resembles meal. They can form large colonies and can be a serious pest.

meiosis: a special type of cell division necessary for sexual reproduction in animals, plants and fungi. The number of sets of chromosomes in the cell undergoing meiosis is reduced to half the original number, typically from two sets (diploid) to one set (haploid). The cells produced by meiosis are either gametes or otherwise usually spores from which gametes are ultimately produced. Since meiosis has halved the number of sets of chromosomes, when two gametes fuse during fertilization, the number of sets of chromosomes in the resulting zygote is restored to the original number..

meristem: is a group of plant cells that can divide indefinitely. The meristem provides new cells for the plant, hence growth. Some meristematic zones are the cambium layer, shoot tips, root tips, and buds.

micropropagation: tissue culture.

mite: a minute arachnid that has four pairs of legs when adult, related to the ticks. Many kinds live in the soil and a number are parasitic on plants or animals. Contrast with insects and nematodes that are not mites.

mitosis: the process by which a cell, which has previously replicated each of its chromosomes, separates the chromosomes in its cell nucleus into two identical sets of chromosomes, each set in its own new nucleus. It is a form of nuclear division. It is generally followed immediately by cytokinesis.

miticide: a pesticide used to kill mites.

mold: a furry growth of minute fungal hyphae occurring typically in moist warm conditions, esp. on food or other organic matter.

morphology: the study of form, especially external structure. See Leaf Morphology.

mucro: a short sharp point at the end of a part.

mucronate: the descriptive term for a leaf ending in a sharp point.

mulch: a material (such as decaying leaves, bark, or compost) spread around or over a plant to enrich and/or insulate the soil.

mutation: an inheritable change in genetic material.

mycelium: the vegetative part of a fungus, consisting of a network of fine white filaments (hyphae).

mycorrhiza, pl. mycorrhizae: any of a group of fungus that forms a symbiotic relationship with roots (or rhizoids), aiding plant nutrition.

N

native: (of a plant or animal) of indigenous origin or growth.

necrotic: dead or dying such as plant tissue.

nectar: a sugary fluid secreted by plants, esp. within flowers to encourage pollination by insects and other animals. It is collected by bees to make into honey.

nectary: a nectar-secreting glandular organ in a flower (floral) or on a leaf or stem (extrafloral).

nematode: a member of the phylum Nematoda, a large phylum of worms with slender, unsegmented, cylindrical bodies, including the roundworms, threadworms, and eelworms. They are found abundantly in soil and water, and many are parasites. Contrast with insects and mites, which are not nematodes.

nervate: leaves with prominent ribs or veins.

node: the part of a plant stem from which one or more leaves emerge, often forming a slight swelling or knob.

noxious weed: a plant that is considered by local authorities to be a problem, growing where it is not wanted. They are often referred to as invasive.

nutrient: a substance that provides nourishment essential for growth and the maintenance of life.

nucleus: a membrane-enclosed organelle found in plant and animal cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are the cell's nuclear genome. The function of the nucleus is to maintain the integrity of these genes and to control the activities of the cell by regulating gene expression — the nucleus is, therefore, the control center of the cell..

nymph: immature form of insects possessing an egg that does not change greatly as it grows, e.g., a dragonfly, mayfly, or locust.

O

obcordate: the descriptive term for a leaf that is heart shaped and the stem is attached to the tapering point. See illustration.

oblanceolate: the descriptive term for a leaf with the top wider than the base.

oblong: much longer than wide, with sides roughly parallel for most of their length.

obovate: the descriptive term for a leaf with a teardrop shape and the stem attaches to the tapering point. See illustration.

obtuse: the descriptive term for a leaf having a blunt end. See illustration.

oomycete, pl. oomycota or oomycetes: ), also known as water molds, any of a distinct fungus-like microorganisms. They have microscopic thread like hyphae just as fungi do, but reproduce both sexually and asexually. Oomycetes occupy both saprophytic and pathogenic lifestyles – and include some of the most notorious pathogens of plants, causing devastating diseases such as Late Blight of Potato and Sudden Oak Death.

opposite: (of leaflets, leaves or shoots) arising in adjacent opposed pairs, one on each side of the stem. See illustration.

orbicular: the descriptive term for a leaf having the shape of a flat ring or disk. See illustration.

order: a principal taxonomic category that ranks below class and above family.

organelle: a specialized subunit within a cell that has a specific function, and it is usually separately enclosed within its own lipid bilayer.

organic: derived from living matter as opposed to inorganic which is not derived from living matter. Coal is organic because it is the carbonized remains of living moss or other plants. However petrified wood is inorganic since the original organic wood is replaced by inorganic calcareous, siliceous, or other mineral deposit. Sometimes organic is defined as "carbon-based".

ovary, pl. ovaries: seed bearing part of the pistil containing the ovules. See flower for a picture.

ovate: the descriptive term for a leaf that is egg shaped with a wide base. See illustration.

ovule: the female reproductive cell of flowering plants and cone-bearing plants. After the ovule is fertilized by the male pollen, the ovule becomes a seed.

ovum, pl. ova: a mature female reproductive cell, esp. of a human or other animal, that can divide to give rise to an embryo usually only after fertilization by a male cell.

P

palmate: the descriptive term for a leaf that is hand-shaped. See illustration. Also, the veins are said to be palmate if several veins diverge from a single point. See illustration.

parallel: in a leaf, the veins run parallel for the length of the leaf, from the base to the apex. See illustration.

parasite: an organism that uses its host for food and/or shelter. A parasite gives its host nothing in return, and often makes it sick or even kills the host. Borers are a parasite of woody plants.

pathogen: an infectious biological agent that causes disease to its host. They include viruses, bacteria, fungi, and oomycetes.

peat moss: a large absorbent moss that grows in dense masses on boggy ground, where the lower parts decay slowly to form peat deposits. Peat moss is widely used in horticulture, esp. for packing plants and (as peat) for compost. Peat moss can acidify its surroundings by taking up cations such as calcium and magnesium and releasing hydrogen ions.

pedate: the descriptive term for a leaf that is palmate with cleft lobes. See illustration.

pedicel: the slender green stalk that connects the base of the flower to a peduncle in a truss composed of several flowers.

peduncle: the spongy material that connects the pedicels of the flowers of a truss together and in turn connects to a stem. This is the part that is severed when a truss is deadheaded. In a plant with individual flowers rather than a truss, a peduncle connects the flower to the stem.

peltate: the descriptive term for a leaf with the stem attached centrally. See illustration.

pendulous: hanging down, drooping.

perennial: living more than two years and fruiting more than once.

perfoliate: the descriptive term for a leaf in which the stem passes through the leaf. See illustration.

petal: each of the segments of the corolla of a flower, which are modified leaves and are typically colored.

petiolated: the descriptive term for a leaf with a petiole. Not sessile. See illustration.

petiole: the stalk connecting a leaf to a stem.

pH: a figure expressing the acidity or alkalinity of a solution on a logarithmic scale on which 7 is neutral, lower values are more acidic, and higher values more alkaline.

phloem: the vascular tissue in vascular plants that conducts sugars and other metabolic products downward from the leaves. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word phloios meaning "bark". The phloem is concerned mainly with the transport of soluble organic material made during photosynthesis. This is called translocation.

photosynthesis: the process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. Photosynthesis in plants generally involves the green pigment chlorophyll and generates oxygen as a byproduct.

phylum, pl. phyla: a principal taxonomic category in zoology that ranks above class and below kingdom. In botany, the equivalent category is division.

pinching: the removing of the terminal bud or growth to encourage branching and denser growth habit.

pinnate: in a leaf, the secondary veins are paired oppositely. See illustration.

pinnate-even: the descriptive term for a compound leaf that is divided with leaflets opposite with two at the tip. See illustration.

pinnate-odd: the descriptive term for a compound leaf that is divided with leaflets opposite with one at the tip. See illustration.

pinnatisect: in a leaf, having deep opposite lobing. See illustration.

pistil: the female organ of a flower consisting of ovary, style and stigma.

plant: a living organism of the kind exemplified by trees, shrubs, herbs, grasses, ferns, and mosses, typically growing in a permanent site, absorbing water and inorganic substances through its roots, and synthesizing nutrients in its leaves by photosynthesis using the green pigment chlorophyll.

plastered indumentum: flattened hairs with a somewhat shiny appearance (see indumentum).

plumule: rudimentary shoot or stem of an embryo plant.

pollen: a fine powdery substance, typically yellow, consisting of microscopic grains discharged from the anther, the male part of a flower, or from a male cone. Each grain contains a male gamete that wind, insects, or other animals can transport to the female where it can fertilize the ovule. Pollen grain is the singular form of pollen.

pollinate: convey pollen to or deposit pollen on a stigma, ovule, flower, or plant and so allow fertilization.

polyploid: describing a cell that has more than two paired sets of chromosomes. Also, describing a plant with such cells. Most organisms have cells that are diploid, i.e. having two sets of chromosomes. Polyploidy is not unusual in plants including rhododendrons and azaleas. Scientists have ways to induce polyploidy in diploids. See triploid, tetraploid and hexaploid.

precocious: flowering before the leaves appear.

proboscis: (in many insects) an elongated sucking mouthpart that is typically tubular and flexible.

propagate: (of a plant, animal, etc.) reproduce by sexual or, more commonly, by asexual means such as cuttings, layering, grafting, or tissue culture.

prune: trim or reduce the length of (something, esp. branches of shrub by plants) by using a sharp implement.

pubescent: with hairs (indumentum and tomentum) or scales, hence with trichomes of any kind.

pungent (leaf): the descriptive term for a leaf having hard sharp points.

pustule: small raised spot or rounded swelling, esp. one on a plant resulting from fungal infection.

Q

quadrate: square or nearly square.

R

racemose: (of a flower cluster) taking the form of a raceme that is a flower cluster with the separate flowers attached by short equal stalks at equal distances along a central stem. The flowers at the base of the central stem develop first.

radicle: the part of a plant embryo that develops into the primary root.

receptacle: the thickened part of a stem from which the flower organs grow. In some fruits, for example the pome and strawberry, the receptacle gives rise to the edible part of the fruit. The fruit of Rubus species is a cluster of drupelets on top of a conical receptacle. When a raspberry is picked, the receptacle separates from the fruit, but in blackberries, it remains attached to the fruit.

recurved: curved back or downward.

reniform (leaf): the descriptive term for a leaf that is kidney-shaped. See illustration.

reticulate: in a leaf, with smaller veins forming a network. See illustration.

retuse (leaf): the descriptive term for a leaf with a shallow notch in a broad apex.

reproduction: the production of offspring by a sexual or asexual process. In plants it is more commonly called plant propagation.

revolute: rolled back or downward from the margins.

rhomboid (leaf): the descriptive term for a leaf with a diamond shape. See illustration.

rhizome: a thick, horizontal underground stem (not a root) of a plant, that grows close to the ground. Rhizomes have nodes and scale-like leaves; roots form on the lower surface and new shoots can form at nodes. Ferns, mosses, horsetails, ginger, irises, and some grasses have rhizomes.

rhododendron: a member of the genus Rhododendron which are shrubs or small trees of the heath family, with large clusters of bell-shaped flowers and typically with large evergreen leaves, widely grown as an ornamental rhododendron (from Greek rhodo meaning rose and dendron meaning tree). Rhododendrons are divided into groups: azaleas, elepidotes, lepidotes and vireyas. Some azaleas are deciduous. The genus Rhododendron is part of the family Ericaceae, order Ericales, subclass Dilleniidae, class Magnoliopsida, division Magnoliophyta, superdivision Spermatophyta, subkingdom Tracheobionta, and kingdom Plantae.

RNA: (short for ribonucleic acid) is a complex, organic, single-stranded molecule that is found in cells. RNA is created by enzymes based on pattern in DNA. RNA carries DNA's instructions for chemical synthesis (like protein formation) to the cell and guides the formation of these chemicals. There are many types of RNA that have different functions. The major types of RNA are messenger RNA (mRNA, which carries the information for protein synthesis from the chromosomal DNA to the ribosomes), transfer RNA (tRNA, which translates mRNA and bonds with amino acids to correctly form the desired proteins), and ribosomal RNA (rRNA, found in ribosomes).

root: a plant structure that obtains food and water from the soil, stores energy, and provides support for the plant. Most roots grow underground.

root collar: the crown, the place where the roots end and the stem begins. When transplanting, the root collar should never be planted any deeper than it was originally growing since this may cause root collar rot.

rootstock: the under-part of a grafted union. See grafting.

rosette: the descriptive term for a compound leaf with leaflets in tight circular rings. See illustration.

rotate: in a peltate leaf, the veins radiating from a central point. See illustration.

rugose: wrinkled.

rugulose: lightly wrinkled.

rufous: reddish.

S

sagittate (leaf): the descriptive term for a leaf with an arrowhead shape.

sap: plant sap, or simply sap, is a fluid transported in xylem cells or phloem vascular elements of a plant. In xylem cells the sap is water and nutrients. In phloem the sap is sugars and other metabolic products.

saprophyte: a plant that secures its food from dead organic matter. They are generally harmless and sometimes beneficial.

scale insect: a small insect with a protective shield-like scale. It spends most of its life attached by its mouth to a single plant, sometimes occurring in such large numbers that it becomes a serious pest. Superfamily Coccoidea, suborder Homoptera: several families, in particular Coccidae.

scales: very tiny, mushroom shaped glandular structures appearing on leaves and other plant parts. Rhododendrons with scales are called lepidotes as opposed to elepidotes. Plants with scales usually do not have hairs or indumentum. Some lepidotes, such as R. edgeworthii, also have indumentum which may obscure the scales leading them to be mistaken for elepidotes occasionally. Scales are a type of trichome.

scion: the shoot or bud grafted on to a rootstock. See grafting

section: a division of classification. Sections and subsections are taxonomic groups between genus and species.

seed: the reproductive unit of some plants. It is formed from a fertilized ovule.

self-sterile: unable to develop seed from its own pollen.

semi-double: a flower type in which some stamens have been transformed completely or partially into petals, usually smaller than the outside corolla petals, and commonly contorted.

semi-double hose-in-hose: a flower type in which the calyx is transformed into petals, such that it appears as two similar corollas, one inside the other and rotated so that all the petals are visible AND some stamens have been transformed completely or partially into petals, usually smaller than the outside corolla petals, and commonly contorted.

sepal: each of the parts of the calyx of a flower, enclosing the petals and typically green and leaf-like.

series: an obsolete division of classification. Series was a taxonomic groups between section and species. Now, subsection is the favored term.

serrate: the descriptive term for a leaf having pointy teeth at the margins that point toward the tip of the leaf. See illustration.

serrulate: the descriptive term for a leaf having pointy teeth with fine serrations at the margins that point toward the tip of the leaf. See illustration.

sessile: the descriptive term for a leaf without a petiole. The blade attaches directly to the stem. Not petiolated. Most grasses are sessile. See illustration.

sexual reproduction: the normal reproduction where pollen from one flower fertilizes the ova in an ovary in the same flower or a different flower causing seed to be formed which inherits traits of both the pollen parent and the "seed" parent. Also see fertilize.

shade: comparative darkness and coolness caused by shelter from direct sunlight.

shrub: a woody plant that is smaller than a tree and has several main stems arising at or near the ground.

shoot: is one of two primary sections of a plant; the other is the root. The shoot refers to what is generally the upper portion of a plant, and consists of stems, leaves, flowers, and fruits.

simple (leaf): the descriptive term for a leaf blade that has one continuous section, not divided into leaflet. Not compound.

single: a single flower on a rhododendron has typically 5 petals fused at the base with a green calyx, a pistil and 5 to 10 stamens.

sinuate: the descriptive term for a leaf having a smooth edges with wave-like indentations. See illustration.

soil: the upper layer of earth in which plants grow, a black or dark brown material typically consisting of a mixture of organic remains, clay, and rock particles.

spatulate: the descriptive term for a leaf having a spoon shape. See illustration.

spear-shaped: the descriptive term for a leaf having a spear shape with a point and a barbed base. See illustration.

species: the principal natural taxonomic unit, ranking below genus and denoted by a Latin binomial, e.g., Rhododendron maximum. The species is a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding.

sphagnum peat: a plant of a genus that comprises the peat mosses.

spider: a flower type in which the petals are narrow and strap-like rather than being fused at the base.

spindly: a plant that is gawky and ungainly because it grows tall with no fullness.

spiny: the descriptive term for a leaf having a sharp stiff points on the margins. See illustration.

spore: a minute, typically one-celled, reproductive unit capable of giving rise to a new individual without sexual fusion, characteristic of lower plants, fungi, and protozoans.

sport: a naturally occurring mutation which usually appears spontaneously with characteristics which differ from the rest of the plant, such as different flower color, variegated leaves, etc.

spotting: dots or spots, usually of darker color, sometimes found on the upper petal(s) of some rhododendrons and azaleas.

stamen: the male fertilizing organ of a flower, typically consisting of a pollen-containing anther and a filament. See flower for a picture.

stem: the main body or stalk of a plant or shrub, typically rising above ground but occasionally subterranean or the stalk supporting a fruit, flower, or leaf, and attaching it to a larger branch, twig, or stalk.

stigma: the tip of the pistil, usually sticky, which receives the pollen for fertilization of the ovule. See flower for a picture.

stock: the under part of a grafted union. It can be either a rooted cutting or a cutting that will be rooted and grafted simultaneously.

stock plant: a plant from which cuttings are taken.

stolon: a creeping horizontal plant stem or runner that takes root at points along its length to form new plants. Plants with stolons are called stoloniferous.

stoma, pl. stomata: a pore (or opening) in a plant's leaves. Most of the stoma are on the underside of the leaf. Guard cells open and close the stoma using turgor pressure, controlling the loss of water vapor and other gases from the plant.

strigose: rough with straight, short sharp appressed hairs.

style: (in a flower) a narrow, typically elongated extension of the ovary, bearing the stigma. See flower for a picture.

subgenus, pl. subgenera: a taxonomic category that ranks below genus and above species.

subspecies: a taxonomic category that ranks below species and above variety, usually a fairly permanent geographically isolated race. Subspecies are designated by a Latin trinomial, e.g., (in zoology) Ursus arctos horribilis or (in botany) Beta vulgaris subsp. crassa.

subobtuse (leaf): the descriptive term for a leaf that is somewhat blunted, neither blunt nor sharp.

suborbicular (leaf): the descriptive term for a leaf that is not quite circular.

subsection: see section.

subulate (leaf): the descriptive term for a leaf that is awl-shaped with a tapering point. See illustration.

suckering: the production of adventitious shoots with elongate internodal lengths and intervals from near or below ground level. They are usually removed. In grafted plants these are from the rootstock and must be removed to save the graft.

sweeten: (of soil) reduce acidity or make neutral or alkaline.

symbiosis: interaction between two different organisms living in close physical association to the advantage of both.

T

taxonomy: the branch of science concerned with classification, esp. of organisms; systematics.

temperate: of, relating to, or denoting a region or climate characterized by mild temperatures.

tender: not hardy, usually needing frost-free conditions and, sometimes, partial shade.

terminal bud: a bud that grows at the end of a stem or shoot. Often contrasted with axillary bud .

ternate (leaf): the descriptive term for a leaf that is divided into three leaflet. Also see trifoliate.

tetraploid: describing a cell that has four paired sets of chromosomes. Most organisms have cells that are diploid, i.e. having two sets of chromosomes. Also, a plant having tetraploid cells. In plant breeding, the induction of tetraploids is a common technique to overcome the sterility of a hybrids. Tetraploid plants in general are more robust and sturdy than diploids.

thrip: a minute black winged insect that sucks plant sap and can be a serious pest of ornamental and food plants when present in large numbers. Order Thysanoptera

throat: the base opening of the corolla.

tissue culture: or micropropagation is propagation by cell division.

tomentose: covered with matted, wooly hair. On rhododendrons, having tomentum or indumentum.

tomentum, pl. tomenta: matted wooly hairs on stems, leaves or seeds. On rhododendrons it usually refers to loose matted wooly hairs of the upper surface of new leaves that usually wears off the first year the leaf comes out, while a hairs on the back side that aren't loose would be called indumentum. As the summer progresses, tomentum gradually vanishes to reveal glossy, deep green leaf surfaces. A type of trichome.

toothed: a type of leaf with jagged edges. See crenate, dentate, denticulate, doubly serrate, serrate, and serrulate.

trace elements: chemical elements that are plant nutrients but are only required in minute quantities.

translocation: transport of food or pesticides through phloem or xylem

transpiration: the process in which plants lose water through pores in their leaves (these openings are called stomata). As water is lost from the plant, the plant takes up more water (and minerals) through its roots. The rate of transpiration varies as the conditions of the plant change and is controlled by the opening and closing the stomata.

transplant: move or transfer to another place or situation, typically with some effort or upheaval. replant (a plant) in another place.

tree: a tall, woody perennial plant usually with a single trunk.

trichome: hair or scales that grow on leaf surfaces and other plant surfaces. On rhododendron, trichomes are called tomentum, indumentum and scales. To a botanist, plants with trichomes of any kind are said to be pubescent.

trifoliate (leaf): the descriptive term for a compound leaf that is divided into three leaflet. See illustration.

tripinnate (leaf): the descriptive term for a compound leaf that is pinnately compound in which each leaflet is also bipinnate. See illustration.

triploid: describing a cell that has three paired sets of chromosomes. Most organisms have cells that are diploid, i.e. having two sets of chromosomes. Also, describing a plant with triploid cells.

truncate (leaf): the descriptive term for a leaf with a squared-off apex. See illustration.

truss: compact cluster of flowers or fruit growing on one stalk and from one bud. Most rhododendrons bear their flowers in trusses. Many azaleas do not.

U

understock: the under part of a grafted union. It can be either a rooted cutting or a cutting that will be rooted and grafted simultaneously.

understory: a layer of vegetation beneath the main canopy of a forest.

undulate (leaf): the descriptive term for a leaf with widely wavy margins. See illustration.

unifoliate: with a single leaf. See illustration.

union: the area where the stock and scion meet and join in grafting.

USDA Hardiness Zone: see Hardiness Zone.

V

variety: a taxonomic category that ranks below subspecies (where present) or species, its members differing from others of the same subspecies or species in minor but permanent or heritable characteristics. Varieties are more often recognized in botany, in which they are designated in the style Apium graveolens var. dulce. Its members differing from others of the same subspecies or species in minor but permanent or heritable characteristics.

vegetative reproduction: reproduction other than by seed.

vein: a vascular bundle in a leaf blade or petal.

vireya: a large group of tropical Rhododendron species and their hybrids which are typically brightly colored but very tender. Vireya have minutes scales on their leaves and are hence lepidotes.

virus: an infective agent that typically consists of a nucleic acid molecule in a protein coat, is too small to be seen by light microscopy, and is able to multiply only within the living cells of a host.

vole: a small, typically burrowing, mouse-like rodent with a rounded muzzle. They are also called meadow mouse or field mouse. They can be destructive when they eat roots or the bark on roots.

W

water sprouts: long fast growing shoots that are frequently undesirable.

weed: a plant growing where it is not wanted and in competition with cultivated plants.

weevil: a small beetle with an elongated snout, the larvae of which typically develop inside seeds, stems, or other plant parts. Also called snout beetle . Curculionidae and other families in the superfamily Curculionoidea

whorled: the descriptive term for a compound leaf with three or more leaflets. See illustration. Also with three or more branches at a node; circle of flower parts as sepals, petals or stamens. See illustration.

woody stem: the persistent main stalk of a shrub, typically rising above ground. The outer layer is the cortex, which may be covered with a corky bark. Underneath are the phloem and the green cambium layer. Inside is the active xylem layer and pervious annual rings of xylem. In the center is a region called the pith.

woolly: clad with long, soft more or less or matted hairs.

X xylem: the vascular tissue in plants that conducts water and dissolved nutrients upward from the root and also helps to form the woody element in the stem. The best-known xylem tissue is wood, though xylem tissue is found throughout the plant. Its basic function is to transport water, but it also transports some nutrients. Compare with phloem .
Y yak: short for R. yakushimanum and its hybrids. R. yakushimanum is now called R. degronianum ssp. yakushimanum. Rhododendron yakushimanum was first discovered in the early 1900s on Yakushima (Yaku island, shima being Japanese for island), a small windswept mountainous island off the south coast of Japan. It is an ideal plant for gardens on lime-free soil. It can be grown in many different situations including full sun, is fully hardy, has a round compact habit and, as it is relatively slow-growing, remains small over many years
Z

zoology: the scientific study of the behavior, structure, physiology, classification, and distribution of animals.

zygote: a diploid cell formed by the union of two gametes (it results from the union of an egg cell (ovule) and a pollen grain). See fertilize.

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