Page:The New International Encyclopædia 1st ed. v. 18.djvu/641

* STELVIO. 547 STEM. was completed by the Austrian Governinent in 1820-24, at an expense of 3,000,000 tiorins. STEM (AS. stcDin, OllG. stain, Ger. Stamm, stem; connected with Olr. tamon, stem, and ulti- mately with Eng. stand). In ".'encral, stems of plants are axes distinguished from roots by bear- ini; leaves or leaf-like organs. Although there are stem-like structures among the thallophytes and bryophytes, it is only among vascular plants (pteriiiophytes and spermatophytes) that real stems appear. Stems may be variously classified. By duration, they are annual or perennial ; by structure, her- baceous or woody: by direction, erect, decumbent, prostrate, climbing, etc. Perhaps one of the most important classifications is upon the basis of character of foliage organs produced, as fol- lows: Foliage-bcnrinfj stems, which generally give style to the whole plant body, are the most conspicuous, since they display green leaves, and are necessarily aerial. As a consequence, they are inclined to branch, and the leaf-bearing joints are well separated. Scale-hearing stems, which bear the comparatively small and colorless (not green) leaf-like bodies (scales), may be subter- of stems, especially those bearing foliage and llowers, is a consi)icuous feature. (For structural details of stems, see Anatosiy of Plants; His- TOLOOY.) Certain broad outlines must be pre- sented here as a basis of the most fundamental classification. At the tip of the stem there is a single apical cell (some pteridophytes) or a group of apical cells, which by their power of continuous division increase the stem in length and give rise to all the tissues. Just behind this growing tip the three great regions be- gin to be defined (Fig. 2). On the outside is a layer of cells (dermatogcn) that gives rise to the epidermis. In perennial stems that increase in diameter the epidermis is FlU. 1. UNDEnRROUND STE.M OF 80L0M0N'B-8EAL. ranean or aerial^ and the joints may be so near together that the scales overlap, as in the aerial scale bud of shrubs and trees, or the subterranean bud-like structures called bulbs, whose scales and stem become gorged with reserve food. Other prominent subterranean types are tubers (potato), comparatively short, thick stems, with much reduced and not overlapping scales, and the more slender rootstocks and rhizomes (Fig. Fig. 2. LONGiTrniNAL skction tbrough apex of stem. Showing dermatogen (d). plerome (p), and between them the periblem. 1 ). Stems hearing flower parts are very much modified, and constitute the so-called flowers of angiosperras (q.v.). The most essential classi- fication of stems is on the basis of their an- atomical structure (see below). The bracing Fig. 3. CROSS-SECTION of a dh-otyledonous stem. Showing pith, three growth rings, and cortex. sloughed off on the older parts, and the region beneath develops a cork tissue of greater or less thickness, which constitutes the bulk of the bark (q.v.). Within the dermatogen is a zone con- sisting of several laj^ers of cells (periblem) that gives rise to the corte.x. Within the periblem is a central solid cylinder of cells (plerome) that Fig. 4. CROSS and longitudinal section of corn stem. Showing the monocittyledonoiis fttructure. gives rise to the stele, characterized by developing the vascular or woody bundles. The arrangement and character of the woody bundles developed in