The Profound Effect on the Structure of Plants 6r 



From the leaf we turn to the associated and well-nigh equally 

 distinctive part, the stem, of which, however, the structure is 

 comparatively simple and uniform. Since its principal function 

 consists in raising and spreading a great many leaves to the light, 

 it must of course be adapted to provide a firm mechanical support 

 in conjunction with much branching; and in fact it consists of 

 a cylindrical-tapering, rigid-continuous, regularly-ramifying struc- 

 ture familiar in the stems of the majority of plants. Although older 

 stems become strongly thickened and woody, and protectively 

 enwrapped in laj^ers of bark, the young growth is soft and green 

 like the leaf, and likewise consists of veins and soft tissue, though 

 the relative importance of the two is reversed in the stem as com- 

 pared with the leaf. The veins can be seen by the eye in young 

 stems that are translucent (e. g., Balsam), when these are held 

 to the light; and they can also be made visible through the tissue 

 in some others if these are stood with their cut ends in a deeply- 

 colored liquid. And they can always be seen in thin sections cut 

 crosswise of the stem, as well illustrated in some later figures 

 (73, 139, B) which accompany a fuller discussion of the stem 

 in another connection. The veins form a ring in most kinds 

 of young stems, though in some they are scattered about; and 

 wherever they branch to run out to the leaves the stem is commonly 

 swollen a httle, and oftentimes lighter in color, giving origin to 

 the so-called nodes separated by spaces called internodes, which 

 are by no means "joints," as sometimes described. Outside 

 the ring of the veins, as the later figures 73 and 141 show very 

 clearly, the soft tissue holds chlorophyll, and thus aids the leaves 

 in their photosynthetic function. The amount of such work 

 that stems can do must in fact be little; but the plant takes ad- 

 vantage, as it were, of every bit of its surface exposed to the light 

 and not needed for other uses, even including such parts as the 

 stamens and pistil of the flower, to spread out additional chloro- 

 phyll for the invaluable photosynthesis. 



Stems, as a rule, grow continuously from buds at their tips, 



