. CHAPTER TEN 



STEMS AND THEIR EXTERNAL FEATURES 



THE stem usually forms the axis of the plant and bears the 

 leaves, flowers, and fruits. Plants showing all degrees of 

 stem branching are found, from the unbranched palm and 

 corn to the finely divided asparagus and elm. In most 

 plants the stems are upright, aerial structures ; but in 

 some plants they lie on the surface of the soil, in other plants 

 the main stem is underground and only the branches rise 

 above the surface, and still other plants have the entire stem 

 underground. The upright stem is the common type and has 

 many advantages over a horizontal stem. At its lower end 

 it bears one or more roots that connect the plant with the soil. 



Advantages of upright stems. 

 The photographer uses light to 

 effect chemical changes in pho- 

 tographic papers and plates, 

 and the plant uses light to bring 

 about chemical changes in its 

 green tissues. The photog- 

 rapher who uses sunlight for 

 his work usually locates his 

 studio at the top of a tall build- 

 ing, because there he avoids 

 the shadows of near-by build- 

 ings and secures a more con- 

 stant exposure to light. The 

 same advantages come to the 

 plant that has its leaves raised 

 well above surrounding plants : 

 the leaves are in less danger of 



FIG. 61. Sunflower and burdock, show- 

 ing advantage of upright stem in leaf 

 display. 



102 



