23.
dioecious plant
dioecious plant is a plant in which the male and female reproductive structures are found in different individuals, as distinct from a monoecious plant, in which they are found in the same individual.
24.
epiphyte
epiphyte is also known as an air plant, any plant that does not normally root in the soil but grows upon another living plant while remaining independent of it except for support (thus differing from a parasite). An epiphyte manufactures its own food in the same way that other green plants do, but obtains its moisture from the air or from moisture-laden pockets of the host plant, rather than from the soil. Some epiphytes are found in every major group of the plant kingdom. Of the flowering plants, the best-known epiphytes are orchids and bromeliads, such as Spanish moss. Epiphytes may grow upon the trunk, branches, or leaves of the host plant, sometimes so thickly as to damage the original plant by crowding out its leaves. They are most abundant in the moist tropics.
25.
flower
Name for the specialized part of a plant containing the reproductive organs, applied to angiosperms only. A flower may be thought of as a modified, short, compact branch bearing lateral appendages. Like twigs, flowers develop from buds, and the basic floral parts (sepal, petal, stamen, and carpel) are in actual fact greatly modified leaves. A typical flower is a concentric arrangement of these parts attached at their base to the receptacle, the tip of the stem. Outermost is a whorl of leaflike green sepals (the calyx) encircling a whorl of usually showy, colored petals (the corolla). Within the corolla the stamens, bearing anther sacs full of pollen, surround the central carpels (ovary). Inside the ovary at the base of the pistil are the ovules, containing the female sex cells; after fertilization of the egg, the ovule becomes the seed and the ovary becomes the fruit. The ovary and stamens are termed essential flower parts, the petals and sepals accessory parts. The number and arrangement of the floral organs vary considerably among the many families and orders of plants and are used in the classification of plants; they also indicate the degree of evolution of the plant. In general, the higher a plant is on the evolutionary scale, the greater is the flower's complexity. The basic number of parts differs from class to class and from family to family; in monocotyledonous plants the parts generally occur in groups of three or in multiples of three, and in dicotyledons more often in groups of two, four, and five. Flowers may be staminate (lack carpels), carpellate, or both; staminate and carpellate flowers may appear on the same plant, on separate plants, or in the same inflorescence. One type of inflorescence, characteristic of the parsley family, is the umbel, in which the tiny florets are borne on separate stalks radiating out from the stem tip. Sometimes the parts serve unusual purposes: the true flowers of the dogwood and the poinsettia are inconspicuous, and the showy “petals” are really modified leaves called bracts. In the jack-in-the-pulpit the florets are clustered on a spike canopied by a large bract, the spathe; the hood of the lady's-slipper, on the other hand, is a modified sterile stamen. Grass inflorescences are tiny spikelets sheathed by protective scales called glumes (the chaff or grain). Flowers have been cultivated and bred for their beauty and their perfume from earliest times and have accumulated a vast and intricate treasury of symbolic associations derived from legend and folklore. Individual flowers have been celebrated in heraldry (rose), in religion (lotus), and in politics (violet) and have become emblems for many countries, including Switzerland (edelweiss), France (fleur-de-lis), Scotland (thistle), Holland (tulip), and the United States.
26.
fruit
fruit, matured ovary of the pistil of a flower, containing the seed. After the egg nucleus, or ovum, has been fertilized and the embryo plantlet begins to form, the surrounding ovule develops into a seed and the ovary wall (pericarp) around the ovule becomes the fruit. The pericarp consists of three layers of tissue: the thin outer exocarp, which becomes the “skin”; the thicker mesocarp; and the inner endocarp, immediately surrounding the ovule. A flower may have one or more simple pistils or a compound pistil made up of two or more fused simple pistils (each called a carpel); different arrangements give rise to different types of fruit. A new variety of fruit is obtained as a hybrid in plant breeding or may develop spontaneously by mutation.
Types of Fruits
Fruits are classified according to the arrangement from which they derive. There are four types—simple, aggregate, multiple, and accessory fruits. Simple fruits develop from a single ovary of a single flower and may be fleshy or dry. Principal fleshy fruit types are the berry, in which the entire pericarp is soft and pulpy (e.g., the grape, tomato, banana, pepo, hesperidium, and blueberry) and the drupe, in which the outer layers may be pulpy, fibrous, or leathery and the endocarp hardens into a pit or stone enclosing one or more seeds (e.g., the peach, cherry, olive, coconut, and walnut). The name fruit is often applied loosely to all edible plant products and specifically to the fleshy fruits, some of which (e.g., eggplant, tomatoes, and squash) are commonly called vegetables. Dry fruits are divided into those whose hard or papery shells split open to release the mature seed (dehiscent fruits) and those that do not split (indehiscent fruits). Among the dehiscent fruits are the legume (e.g., the pod of the pea and bean), which splits at both edges, and the follicle, which splits on only one side (e.g., milkweed and larkspur); others include the dry fruits of the poppy, snapdragon, lily, and mustard. Indehiscent fruits include the single-seeded achene of the buttercup and the composite flowers; the caryopsis (grain); the nut (e.g., acorn and hazelnut); and the fruits of the carrot and parsnip (not to be confused with their edible fleshy roots).
An aggregate fruit (e.g., blackberry and raspberry) consists of a mass of small drupes (drupelets), each of which developed from a separate ovary of a single flower. A multiple fruit (e.g., pineapple and mulberry) develops from the ovaries of many flowers growing in a cluster. Accessory fruits contain tissue derived from plant parts other than the ovary; the strawberry is actually a number of tiny achenes (miscalled seeds) outside a central pulpy pith that is the enlarged receptacle or base of the flower. The core of the pineapple is also receptacle (stem) tissue. The best-known accessory fruit is the pome (e.g., apple and pear), in which the fleshy edible portion is swollen stem tissue and the true fruit is the central core. The skin of the banana is also stem tissue, as is the rind of the pepo (berrylike fruit) of the squash, cucumber, and melon.
The Role of Fruits in Seed Dispersal
The structure of a fruit often facilitates the dispersal of its seeds. The “wings” of the maple, elm, and ailanthus fruits and the “parachutes” of the dandelion and the thistle are blown by the wind; burdock, cocklebur, and carrot fruits have barbs or hooks that cling to fur and clothing; and the buoyant coconut may float thousands of miles from its parent tree. Some fruits (e.g., witch hazel and violet) explode at maturity, scattering their seeds. A common method of dispersion is through the feces of animals that eat fleshy fruits containing seeds covered by indigestible coats.
27.
gametophyte
A phase of plant life cycles in which the gametes, i.e., egg and sperm, are produced. The gametophyte is haploid, that is, each cell contains a single complete set of chromosomes, and arises from the germination of a haploid spore. In many lower plants, the gametophyte phase is the dominant plant form; for example, the familiar mosses are the gametophyte form of the plants. The alternate phase of the plant life cycle is the sporophyte, the diploid plant form, with each cell containing two complete sets of chromosomes. For example, in mosses the sporophyte is a capsule atop a slender stalk that grows out of the top of the gametophyte. The sporophyte develops from the union of two gametes, such as an egg fertilized by a sperm; in turn, the sporophyte forms spores that develop into gametophytes. The alternation between haploid gametophyte and diploid sporophyte phases, known as alternation of generations, occurs in all multicellular plants. As plants advanced in evolutionary development, the sporophyte became the increasingly dominant plant form and the gametophyte form has been correspondingly reduced. In contrast to mosses, for example, in the advanced angiosperms the male and female gametophytes are reduced to three-celled and seven-celled structures, respectively, found within the reproductive organs of the familiar flowering plant (the sporophyte).
28.
germination
germination, in a seed, process by which the plant embryo within the seed resumes growth after a period of dormancy and the seedling emerges. The length of dormancy varies; the seed of some plants (e.g., most grasses and many tropical plants) can sprout almost immediately, but many seeds require a resting stage before they are able to germinate. The viability of seeds (their capacity to sprout) ranges from a few weeks (orchids) to over 400 years (Indian lotus) and up to 10,000 years (Arctic lupine). The percentage of viable seed decreases with age. Dormancy serves to enable the seed to survive poor growing conditions; a certain amount of embryonic development may also take place. Dormancy can be prolonged by extremely tough seed coats that exclude the water necessary for germination. Internally, growth is regulated by hormones called auxins. When the temperature is suitable and there is an adequate supply of moisture, oxygen, and light—although some seeds require darkness and others are unaffected by either—the seed absorbs water and swells, rupturing the seed coat. The growing tip (radicle) of the rudimentary root (hypocotyl) emerges first and then the growing tip (plumule) of the rudimentary shoot (epicotyl). Food stored in the endosperm or in the cotyledons provides energy for the early stages of this process, until the seedling is able to manufacture its own food via photosynthesis.
29.
gibberellins
a group of growth-regulating substances of plants, having complex chemical structure, of which the best known, gibberellic acid, is noted for its promotion of stem growth. In Japan it was long known that when rice seedlings were attacked by the fungus Gibberella fujikuroi they would grow to several times their normal height and then die, a phenomenon the Japanese called “the foolish seedling disease.” A substance that caused these same effects was isolated from the fungus and named gibberellin. Other gibberellins exist rather widely in plants, and only an excess appears to cause abnormal effects. Gibberellins are used commercially in agriculture and horticulture to break dormancy, to speed up flowering and fruiting, and to stimulate the production of seedless fruits in the absence of pollination.
30.
growing season
growing season is the period during which plant growth takes place. In temperate climates the growing season is limited by seasonal changes in temperature and is defined as the period between the last killing frost of spring and the first killing frost of autumn, at which time annual plants die and biennials and perennials cease active growth and become dormant for the cold winter months. In tropical climates, in which there is less seasonal temperature change, the amount of available moisture often determines the periods of plant growth; in the rainy season growth is luxuriant and in the dry season many plants become dormant. In desert areas, growth is almost wholly dependent on moisture. In the Arctic the growing season is short but concentrated; the number of daylight hours is so large that the total amount of sunlight equals that of a temperate growing season with shorter days. The length of the growing season often determines which crops can be grown in a region; some require long growing seasons and others mature rapidly. Plants that are perennials in a warm climate may sometimes be grown as annuals in cooler areas; by crossing hardy plant species with less hardy but more productive types, plant breeders have developed desirable new strains that mature in a shorter period. Combinations of factors affect the growing season; in the sheltered valleys and coastal slopes of the Pacific Northwest of the United States, the heavy winter rainfall and the dry summers have produced a Mediterranean type of climate where plant growth occurs during the winter and dormancy during the summer.
31.
halophyte
any plant, especially a seed plant, that is able to grow in habitats excessively rich in salts, such as salt marshes, sea coasts, and saline or alkaline semideserts and steppes. These plants have special physiological adaptations that enable them to absorb water from soils and from seawater, which have solute concentrations that nonhalophytes could not tolerate. Some halophytes are actually succulent, with a high water-storage capacity.
32.
heartwood
heartwood, the central, woody core of a tree, no longer serving for the conduction of water and dissolved minerals; heartwood is usually denser and darker in color than the outer sapwood. Before the synthesis of aniline dyes, the heartwood of several tropical trees
(sold collectively under the commercial name brazilwood) was used to produce blue, purple, and red dyes. As a tree becomes older, the heartwood increases in diameter, whereas the sapwood remains about the same thickness.
33.
herb
It is a name for any plant that is used medicinally or as a spice and for the useful product of such a plant. Herbs as condiments and seasonings are still important in culinary art; the use of medicinal herbs, however, has waned since the advent of prescription and synthetic medicines, although plants remain a major source of drugs. The term herb is also applied to all herbaceous plants as distinguished from woody plants.
34.
herbaceous plant
A plant whose stem is soft and green and shows little growth of wood. The term is used to distinguish such plants from woody plants. Herbaceous plants, or herbs, as they are commonly called, may be annual—that is, the plants die after a year's growth, and the plants are propagated by seed—or they may be produced each year by new shoots from dormant roots. The stems of woody plants, e.g., most shrubs and trees, are tough, are covered with nongreen bark, and enlarge in diameter by the accumulation of annual layers of wood produced by the cambium.
35.
herbarium
The collection of dried and mounted plant specimens used in systematic botany. To preserve their form and color, plants collected in the field are spread flat in sheets of newsprint and dried, usually in a plant press, between blotters or absorbent paper. The specimens, mounted on sheets of stiff white paper, are labeled with all essential data, e.g., date, where found, description of the plant, altitude, special habitat conditions, and placed in a protective case. As a precaution against insect attack the pressed plant is frozen or poisoned and the cases disinfected. Herbariums are essential for the study and verification of plant classification, the study of geographic distributions, and the standardizing of nomenclature. Thus inclusion of as much of the plant (e.g., flowers, stems, leaves, seed, and fruit) as possible is desirable. Linnaeus' herbarium now belongs to the Linnaean Society in England. Most universities maintain herbariums. Notable herbariums in the United States include the Gray Herbarium at Harvard and those at the U.S. National Museum (of the Smithsonian Institution) and at the New York and Missouri botanical gardens.