36. isogamy

a condition in which the sexual cells, or gametes, are of the same form and size and are usually indistinguishable from each other. Many algae and some fungi have isogamous gametes. In most sexual reproduction, as in mammals for example, the ovum is quite larger and of different appearance than the sperm cell. This condition is called anisogamy.





37. kinetin

one of a group of chemically similar plant hormones, the cytokinins, that promote cell division. In some instances kinetin acts together with another hormone, indoleacetic acid, or auxin; in other cases it acts in opposition to auxin.






38. leaf

leaf, chief food-manufacturing organ of a plant, a lateral outgrowth of the growing point of stem. The typical leaf consists of a stalk (the petiole) and a blade—the thin, flat, expanded portion (needlelike in most conifers) that is normally green in color because of the presence of the pigment chlorophyll. In many leaves, small processes called stipules occur at the base of the stalk and protect the bud; sometimes the stipule is large (as in the Japanese quince) and, if green, also manufactures food. The leaf blade is veined with sap-conducting tubes (xylem and phloem) with thick-walled supporting cells. The blade consists of an upper and a lower layer of closely fitted epidermal cells, including specialized paired guard cells that control the size of tiny pores, or stomata, for gaseous exchange and the release of water vapor. The upper epidermis is usually coated with a waterproof cuticle and contains fewer stomata than the underside, if any at all. Between these two layers are large palisade and spongy cells, rich in chlorophyll for food manufacture and permeated with interconnecting air passages leading to the stomata. Leaves vary in size (up to 60 ft/18m long in some palms), shape, venation, color, and texture, and are classified as simple (one blade) or compound (divided into leaflets). The blade margins may be entire (smooth and unindented), toothed (with small sharp or wavy indentations), or lobed (with large indentations, or sinuses). In monocotyledonous plants, the veins are usually parallel; dicotyledons have leaves with reticulately branched veins that may be pinnate (with one central vein, the midrib, and smaller branching veins) or palmate (with several large veins branching from the leaf base into the blade). Pigments besides chlorophyll that give a leaf its characteristic color are the carotenoids (orange-red and yellow), the anthocyanins (red, purple, and blue), and the tannins (brown). White results from the absence of pigments. In deciduous plants, a layer of cells forms the abscission tissue at the base of the stalk in the autumn, cutting off the flow of sap; the unstable chlorophyll disintegrates and, in a temperate zone, the remaining pigments are displayed to produce colorful fall foliage. When these cells dry up completely, the leaf falls. Evergreen plants usually produce new leaves as soon as the old ones fall; the leaves of most conifers remain on the tree from 2 to 10 years (in some species up to 20 years). Leaves may be modified or specialized for protection (spines and bud scales), climbing (tendrils), trapping insects (as in pitcher plants), water storage (as in succulents), or food storage (bulb scales and, in the embryo plantlet, cotyledons).







39. leaf mold

leaf mold, crumbly brown humus typical of forest floors. It is composed of decayed leaves and other plant material mixed with soil.






40. liana

A name for any climbing plant that roots in the ground. The term is most often used for the woody vines that form a characteristic part of tropical rain-forest vegetation; they are sometimes also called bushropes or simply vines. Although lianas are found in every climate where there are trees to support them, they are most abundant and luxuriant in the tropics, where rapid growth to reach the light is of particular advantage in the dense vegetation. There they often ascend and descend more than one tree. Climbing palms have been measured at over 700 ft (210 m) long; a length of over 200 ft (60 m) is not unusual for many other types. Most plant families with tropical species include lianas. The distinction between true lianas and weak-stemmed trees or half-climbing shrubs cannot always be clearly drawn and depends largely on the age of the plant concerned.






41. lignin

a highly polymerized and complex chemical compound especially common in woody plants. The cellulose walls of the wood become impregnated with lignin, a process called lignification, which greatly increases the strength and hardness of the cell and gives the necessary rigidity to the tree. It is essential to woody plants in order that they stand erect.

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