CHAPTER FOURTEEN 



THE STEMS OF PLANTS 



The stem forms the axis of the plant and bears the leaves, 

 flowers, and fruits. Plants showing all degrees of stem branch- 

 ing 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 he on the surface of 

 the soil, in other plants the main stem is underground and only 

 the branches rise above the surface, and in still other plants the 

 entire stem is underground. The upright stem is the common 

 type and has many advantages over a horizontal stem. 



Upright stems. The photographer uses light to effect chemical 

 changes in photographic papers and plates. Light also brings 

 about chemical changes in the green tissues of the plant. The 

 photographer who uses sunlight for his work usually locates his 

 studio at the top of a tall building, because there he avoids the 

 shadows of near-by buildings and secures a more constant ex- 

 posure to hght. The same advantages come to the plant that 

 has its leaves raised well above surrounding plants; the leaves 

 are in less danger of being shaded, and each day they are exposed 

 to the sunshine during a longer period (Fig, 55). 



The tall plant has an additional advantage in being able to 

 expose to the light a greater leaf area over a given space of ground, 

 because it can display several or many layers of leaves one above 

 the other. The rosette of leaves formed by the burdock illus- 

 trates the possibilities of leaf display near the soil. A large sun- 

 flower plant covers no greater soil area than a burdock, but it is 

 able to expose to the sunhght several times as great a leaf area 

 because the sunflower leaves are placed at several different levels. 

 Trees have the greatest stem development and the greatest leaf 

 display. Rosette plants, hke the dandelions and plantains, repre- 

 sent the opposite extreme of slight stem development, small leaf 

 area, and a relatively poor leaf display. One advantage in a tall, 



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