Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

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Multicellular Organisms
Higher Plants and Plant Tissues

Botanists divide higher plants into two main groups. Bryophytes, or mosses and liverworts, consist of mosses (Musci) and liverworts (Hepaticae). These predominantly terrestrial plants are characterized by the presence of motile sperm Cells and the dominance of the gametophytic (haploid) phase. The second group comprises Tracheophytes, or vascular plants, which possess conducting Tissues. This group includes about 2∙10⁵ species. Ferns (Class Filicinea, formerly known as Pteridophyta) are characterized by the dominance of the diploid phase, which alternates with the haploid phase during The life cycle (sec. B.5).

Seed plants are represented by two classes: Gymnospermae (gymnosperms, which include cone-bearing trees) and Angiospermae (angiosperms, the true flowering plants).

Several types of plant tissues are distinguished. Undifferentiated embryonic cells located in the rapidly growing PARTS OF THE stem and ROOT constitute the meristem tissue. As a result of Cell Differentiation from the meristem, simple tissues are formed: parenchyma, collenchyma, and sclerenchyma. Parenchyma is the most common and least specialized type of plant tissue. Further differentiation yields the cambium, the growing layer of roots and stems. Parenchyma also gives rise to cells in which the plant stores nutrients, forming the pith, or the core and cortex of the stem and root.

Collenchyma, found in herbaceous plants, consists of elongated supporting cells; sclerenchyma, typical of trees, is formed by supporting cells with robust lignified walls and a low Water content. Sclerenchyma also includes fibrous cells that can occasionally be very long: for example, the pine trunk contains fibrous cells 40 µm in diameter and 4 mm long.

Two Types of complex tissues—xylem and phloem—form the conducting network, or the "Circulatory system," of the plant. The cells forming the xylem, or woody tissue, are mostly dead, and thick-walled vessels (tracheids) serve to transport water and dissolved minerals from the roots to the stem and leaves. Phloem cells provide the outflow of nutrients from the leaves down the stem. They are connected at their ends by sieve plates perforated by numerous tiny pores, through which the Cytoplasm of adjoining cells apparently communicates, forming strands 5–9 µm in diameter [54]. Mature sieve cells lack a Nucleus, but each has a "partner"—a companion cell containing a nucleus.

FIG. 1-12. Cross-section of an angiosperm stem. The bottom views show magnified cross-sections of phloem vessels (left) and xylem vessels (right) (Biddulph S., Biddulph О., Sсi. Am. 200, 44—49, Feb. 1959.)

The plant epidermis consists of flat cells with thick outer walls, typically covered by a dense waxy cuticle about 2 µm thick. METABOLISM/14.html">Chloroplasts are usually absent in these cells. Only a few types of specialized cells are found in the epidermal tissue. These include paired guard cells that surround small openings called Stomata on the lower surface of the leaf; the guard cells regulate The rate of water Transpiration by the plant. Specialized Cells of the root epidermis form root hairs—long (~1 mm) structures 5–17 µm in diameter. Each root Hair is a single cell with a nucleus located at the tip of the hair.

Fig. 1-12 shows a section of a flowering plant stem. Note the thin layer of cambium between the phloem and xylem. As they grow, cambial cells differentiate and form new layers of xylem, thereby increasing the woody portion of the stem. Phloem cells are also formed simultaneously. As the stem thickens, all tissues located outward from the cambium are renewed, while older cells transform into bark.

Plant seeds consist of three distinct parts. The embryo develops from a zygote formed by the fusion of a sperm nucleus (derived from a pollen grain) with an egg nucleus. In gymnosperms, the fertilized egg is surrounded by a nutritive layer, or endosperm, derived from the same gametophytic tissue as the egg and is therefore haploid. In angiosperms, two nuclei are formed in the sperm: one fertilizes the egg, while the other fuses with two haploid polar nuclei produced in the female gametophyte. (These polar nuclei are formed during the same mitotic division that produces the egg cell.) As a result, a triploid (3n) endosperm develops.