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make up the pith or pulp in the center of stems androots, where they serve as food storage cells.

The collenchyma, present in herbs, is composedof elongated supporting cells and the sclerenchyma

of woody plants is made up of supporting cells withhard lignified cell walls and a low water content.This tissue includes fiber cells, which may be ex-tremely long; e.g., pine stems contain fiber cells of 40m diameter and 4 mm long.

Two complex tissues, thexylem and phloem,provide the conducting network or circulatory systemof plants. In the xylem or woody tissue, most of thecells are dead and the thick-walled tubes (tracheids)serve to transport water and dissolved minerals fromthe roots to the stems and leaves. The phloem cellsprovide the principal means of downward conductionof foods from the leaves. Phloem cells are joined end

to end by sieve plates, so-called because they areperforated by numerous minute pores through whichcytoplasm of adjoining sieve cells appears to be con-nected by strands 59 m in diameter.154 Mature sievecells have no nuclei, but each sieve cell is paired with anucleated companion cell.

Epidermal tissue of plants consists of flat cells,usually containing no chloroplasts, with a thick outerwall covered by a heavy waxy cuticle about 2 m thick.Only a few specialized cells are found in the epidermis.Among them are the paired guard cells that surroundthe small openings known as stomata on the under-surfaces of leaves and control transpiration of water.Specialized cells in the root epidermis form root hairs,long extensions (~1 mm) of diameter 517 m. Eachhair is a single cell with the nucleus located near the tip.

Figure 1-16 shows a section from a stem of a typicalangiosperm. Note the thin cambium layer betweenthe phloem and the xylem. Its cells continuously

undergo differentiation to form new layers of xylemincreasing the woody part of the stem. New phloemcells are also formed, and as the stem expands all ofthe 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 fusionof a sperm nucleus originating from the pollen andan egg cell. The fertilized egg is surrounded in thegymnosperms by a nutritive layer or endospermwhich is haploid and is derived from the same game-tophyte tissue that produced the egg. In angiospermstwo sperm nuclei form; one of these fertilizes the egg,while the other fuses with two haploid polar nucleiderived from the female gametophyte. (The polar nucleiare formed by the same mitotic divisions that formedthe egg.) From this develops a 3n triploid endosperm.

G. The Chemical Composition of Cells

Water is the major component of living cells, butthe amount varies greatly. Thus, the pig embryo is97% water; at birth a new-born pig is only 89% water.A lean 45-kg pig may contain 67% water but a very fat135-kg animal only 40% water. Similar variations areencountered with other constitutents.

The water content of a tissue is often determinedby thoroughly drying a weighed sample of tissue atlow temperature in vacuum and then weighing it asecond time. The solid material can then be extractedwith a solvent that will dissolve out the fatty com-pounds. These are referred to collectively as lipids.After evaporation of the solvent the lipid residue maybe weighed. By this procedure a young leafy vegetablemight be found to contain 25% lipid on a dry weight

Figure 1-16 Section of the stem of an angiosperm. Enlarged sections showing tubes of the phloem (left) and xylem (right).From S. Biddulph and O. Biddulph.155 Drawn by Bunji Tagawa.

RaySievetube

Sieveplate

Fiber

Ray

Fiber

Vessel

Ray

Xylem

Pith

Phloem

Cambium

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basis. Even very lean meats contain 1030% lipid.The residue remaining after removal of the lipid

consists predominately of three groups of compounds:proteins, nucleic acids, and carbohydrates. Most of

the nitrogen present in tissues is found in the proteinsand the protein content is sometimes estimated bydetermining the percentage of nitrogen and multiplyingby 6.25. In a young green plant, 20 30% of the drymatter may be protein, while in very lean meat it mayreach 50 70%.

A dried tissue sample may be burned at a hightemperature to an ash, which commonly amounts to310% and is higher in specialized tissues such asbone. It is a measure of the inorganic constituents of

tissues.The carbohydrate content can be estimated by thedifference of the sum of lipid, protein, and ash from100%. It amounts to 5060% in young green plantsand only 210% in typical animal tissues. In excep-tional cases the carbohydrate content of animal tissuesmay be higher; the glycogen content of oysters is 28%.

The amount of nucleic acid in tissues varies from0.1% in yeast and 0.51% in muscle and in bacteria to1540% in thymus gland and sperm cells. In these lattermaterials of high nucleic acid content it is clear thatmultiplication of % N by 6.25 is not a valid measure ofprotein content. For diploid cells of the body the DNA

content per cell is nearly constant.Table 1-4 compares the composition of a bacterium,of a green plant, and of an active animal tissue (ratliver). Although the solid matter of cells consistsprincipally of C, H, O, N, S, and P, many other chemicalelements are also present. Among the cations, Na+, K+,Ca2+, and Mg2+ are found in relatively large amounts.Thus, the body of a 70 kg person contains 1050 g Ca(mostly in the bones), 245 g K, 105 g Na, and 35 g Mg.Iron (3 g), zinc (2.3 g), and rubidium (1.2 g) are the nextmost abundant. Of these iron and zinc are essential tolife but rubidium is probably not. It is evidently taken upby the body together with potassium.

The other metallic elements in the human bodyamount to less than 1 g each, but at least seven of themplay essential roles. They include copper (100 mg),manganese (20 mg), and cobalt (~5 mg). Others, suchas chromium (