THE STRUCTURE OF THE SHOOT 



69 



vascular strands throughout the plant ensures the proper 

 distribution of the water absorbed from the soil. 



Certain other features of the framework of the plant 

 next call for attention. If we examine the outer layer, 

 called in the stem and leaf the epidermis, we find it as a 

 continuous sheet over the whole, and in most cases a 



FIG. 27. Sieve tube from stem of Cucurbita. A, transverse; B, longitu- 

 dinal section ; s.p., sieve plate ; c, companion cell. (After Strasburger.) 

 X5oo. 



single cell in thickness. A delicate structure like a seed- 

 ling, whose cells are filled with water, is exposed to a 

 general evaporation at the surface. This, if not guarded 

 against, would lead to a loss of water beyond the control 

 of the plant, and would interfere with the proper con- 

 duction of the water to the places where the construction 

 of sugar takes place. We find a very simple but very 

 effective protective mechanism. The outer walls of the 

 cells of the epidermis become thickened and their 

 external layers are changed into a very impermeable 



