Genetically the simplest of the angiosperms is the little weed Arabidopsis thaliana, whose generation time is as short as five weeks. Its five chromosomes contain only 108 base pairs in all, the smallest known genome among angiosperms153 and one whose complete nucleotide sequence is being determined. Its biochemistry, physiology, and developmental biology are under intensive study. It may become the “fruit fly” of the plant kingdom. There are several kinds of plant tissues. Undifferentiated, embryonic cells found in rapidly growing regions of shoots and roots form the meristematic tissue. By differentiation, the latter yields the simple tissues, the parenchyma, collenchyma, and sclerenchyma. Parenchyma cells are among the most abundant and least specialized in plants. They give rise through further differentiation to the cambium layer, the growing layer of roots and stems. They also make up the pith or pulp in the center of stems and roots, where they serve as food storage cells. The collenchyma, present in herbs, is composed of elongated supporting cells and the sclerenchyma of woody plants is made up of supporting cells with hard lignified cell walls and a low water content. This tissue includes fiber cells, which may be extremely long; e.g., pine stems contain fiber cells of 40 µm diameter and 4 mm long. Two complex tissues, the xylem and phloem, provide the conducting network or “circulatory system” of plants. In the xylem or woody tissue, most of the cells are dead and the thick-walled tubes (tracheids) serve to transport water and dissolved minerals from the roots to the stems and leaves. The phloem cells provide the principal means of downward conduction of foods from the leaves. Phloem cells are joined end to end by sieve plates, so-called because they are perforated by numerous minute pores through which cytoplasm of adjoining sieve cells appears to be connected by strands 5–9 µm in diameter. Mature sieve cells have no nuclei, but each sieve cell is paired with a nucleated“companion” cell. Epidermal tissue of plants consists of flat cells, usually containing no chloroplasts, with a thick outer wall covered by a heavy waxy cuticle about 2 µm thick. Only a few specialized cells are found in the epidermis. Among them are the paired guard cells that surround the small openings known as stomata on the undersurfaces of leaves and control transpiration of water. Specialized cells in the root epidermis form root hairs, long extensions (~1 mm) of diameter 5–17 µm. Each hair is a single cell with the nucleus located near the tip. Figure 1-16 shows a section from a stem of a typical angiosperm. Note the thin cambium layer between the phloem and the xylem. Its cells continuously undergo differentiation to form new layers of xylem increasing the woody part of the stem. New phloem cells are also formed, and as the stem expands all of the tissues external to the cambium are renewed and the older cells are converted into bark.
Plant seeds consist of three distinct portions. The embryo develops from a zygote formed by fusion of a sperm nucleus originating from the pollen and an egg cell. The fertilized egg is surrounded in the gymnosperms by a nutritive layer or endosperm which is haploid and is derived from the same gametophyte tissue that produced the egg. In angiosperms two sperm nuclei form; one of these fertilizes the egg, while the other fuses with two haploid polar nuclei derived from the female gametophyte. (The polar nuclei are formed by the same mitotic divisions that formed the egg.) From this develops a 3n triploid endosperm.
Section of the stem of an angiosperm. Enlarged sections showing tubes of the phloem (left) and xylem (right). From S. Biddulph and O. Biddulph. Drawn by Bunji Tagawa.
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