CHAPTER VI 

 THE STEM AND ITS FUNCTIONS 



We have shown that the root, which absorbs water and 

 mineral substances from the soil, and the leaf, which carries 

 on the manufacture of food, are the primary vegetative organs 

 of the plant. A third member, the stem, connects these two. 

 It forms a conspicuous feature of most plants and in woody 

 species constitutes the great bulk of their bodies. Its functions, 

 though secondary to the major activities which we have 

 mentioned, are nevertheless essential ones. It serves to dispose 

 the leaves in situations favorable for photosynthesis, and provides 

 a highway for transportation between leaf and root. In 

 addition, the stem frequently becomes a storage-organ and may 

 be variously modified for other special functions. 



The External Structure of the Stem. — The stem displays a 

 wide range of variation in size and in external and internal struc- 

 ture, according to the habit or growth-form which the plant 

 assumes. In herbs (Fig. 40), where the whole shoot dies back 

 to the ground during periods unfavorable to vegetative activity 

 or at the completion of a given cycle, the stem is comparatively 

 slender and soft in texture. In plants with perennial above- 

 ground parts, however, it grows thicker from year to year and 

 becomes hard and woody, forming the stout stems characteristic 

 of shrubs (Fig. 41) and trees (Fig. 42). In shrubs the stem is 

 comparatively short and slender and is usually much branched, 

 even close to the ground. In trees, it grows taller and is devel- 

 oped for some distance upward into a main stem or trunk which 

 may become very thick. Woody stems transitional between these 

 two types often occur. 



Buds. — The growth of the stem in length takes place only at 

 certain definite points, where the cells are thin-walled and capable 

 of active division. In many stems, particularly those which are 

 perennial and woody, these growing-points are protected by leaves 

 or scales and are then known as buds (Fig. 43). Buds may be 

 terminal, developing at the tip of the stem, or lateral, arising from 



