Page:The New International Encyclopædia 1st ed. v. 01.djvu/602

ANATOMY or PLANTS. growths, but of varied form, are not infrequently found on other parts of the body. (See HepaTic.E.) In mosses, the body, when young, is a much-branclied filament, usually transient, from which there arises a more permanent cylindrioal axis, erect, with few branches, or horizontal and much branched, on whose sides are developed green thin outgrowths, the leaves. These are usually a single sheet of cells, except near the middle line, where several layers of cells con- stitute a midrib. The sliapes of the leaves are extremely varied. See Mrsci.

PTERIDOPHYTE.S AND SpERMATOPIIYTES. The gametopliyte of the ferns is a thin, roundish, heart-shaped body, seldom exceeding a quarter of an inch in diameter, and transient. In fern allies it is reduced to a few cells, and is not visible without microscopic observation. The vegeta- tive body, which is long lived, does not bear se.x organs as in the lower plants, bvit gives rise only to non-sexual reproductive bodies of various kinds called spores ; whence it is kno^■n as the sporophyte. (See Alternation of Gen- EKATIONS. ) The external anatomy of the sporo- phyte in the pteridophytes (ferns and their allies) and spermatophytes (seed plants or flowering plants) is much alike. It is almost without exception segmented into two distinct parts, the root and the shoot. The root is usually much branclied, and bears on its newer parts sur- face outgrowths called root-hairs. The tips of the branches, at ^■l^ch the growing points are lo- cated, are protected by somewhat older cells, which constitute a root-cap. (See Root.) The shoot is usually dill'erentiated into a central axis, the stem, with lateral outgrowths of two kinds : ( 1 ) Those having unlimited growth : i.e., branches, which are similar in all essential re- spects to the main axis, though they may be spe- cialized in form and function. (For further de- tails respecting the strvicture of the stem and the forms which it assinnes, see Stem.) (2) Seg- ments with limited growth, usually flat, thin, and broad; i.e., leaves, which are developed in a variety of forms. (See Leaf.) Simple out- gi-owths of various forms, arising from single cells or small groups of cells, may develop on any part of the shoot as scales, hairs, etc.

Internal Anatoiiy. The internal anatomy of plants can only be ascertained by the study of thin sections, crossing the body in various planes, and by dissection of the parts. In the lower plants the internal anatomy is as simple as the external form, no differentiation of the cells being observable. In the higher plants, however, groups of cells are difl'ercntiated into tissues, and the tissues are arranged into systems, each of which has a particular function to fulfill. These tissue systems are named according to their function. The most important are the following: (1) The protective system; (2) the absorptive system; (3) the conducting system; (4) the nutritive system; (5) the aerating sys- tem; (0) the secreting system; (7) the storage system; (8) the iliechanical system.

The tissues and tissue systems all arise in an uns])ccialized formative tissue. Every plant be- gins its development as a single cell. Repeated divisions of this cell and its segments give rise to others essentially similar. For a time these cells ret.ain the same general form and powers, noteworthy among which is the capacity of divis- ion. .s the cells become older they grow unlike, and change not only in form but in function. Some maintain throughout their entire existence the form and appearance of the j'oungest cells. These constitute a formative region (meristem), which, by its growth, gives rise constantly to new tis- sues and new organs. This primarj' formative tissue is found in the larger plants at the ex- ti-emities of the main axis and branches of the roots and shoots, where it constitutes the grow- ing points. It is possible, however, for cells which have ceased to divide to regain this power and to resume the character of formative tissue. To distinguish this latter from the pi'imary meri- stem of the growing points, it is called secondary meristem, or cambium. Secondary meristem is often formed in one or more concentric zones in the stems of those plants which increase in thick- ness as they become older, in the vicinity of wounds, and at various ])oints which cannot al- ways be predicted. At these points its activity I'esults in the making of new tissues, or even new organs, in a maner essentially like that in the growing points. A short distance behind each growing point the cylinder of newh' formed tis- sues differentiates enough to show three regions: ( 1 ) The outer cell layer, the "dermatogen," which gives rise to the epidermis, and all its structures; (2) the central mass, the "plerome," which develops the central cylinder or stele, in- cluding the vascular strands and pith; (3) be- tween these, the "periblem.'' which jiroduces the cortex. The distinction lietween these regions is permanent, becoming more marlced with age. ( 1 ) TiiE Protective Sy'steii. In alga; and fungi, which live in water or moist places, the need for protection is seldom of moment. The transformation of the surface walls into a gela- tinous material is common among even the sim- plest algae, and may be looked upon as a protec- tive measure. Some of the larger alga? have the cells near the surface of the body smaller and more compact than those of the interior. In the aerial parts of some fungi a comjjact arrange- ment or coalescence of the filaments, and some- times the thickening of their walls, makes the surface firm or even hard. In the larger plants, however, the surface tissues are usually organized into a continuous membrane, the e])idermis, in places perforated by numerous but minute openings, the stomata, which are bounded by special cells, and guard cells. (See Stomata.) The epi-

Oork cells ipfridfrin) developed under the epiderniii* (the outer layer of cells) by divisions ijariillel to the Burface iu the cork canibiuiji iphcUogen), next the rounded cells of the cortex.

dermis sometimes becomes more than one layer j of cells in thickness. This is regularly the ease] at the tip of the root, where it forms a thimble- shaped cap. The outer wall of the epidermal I cells is frequently much thickened, and is usu- ally partially occupied by a wax-like substance,]