40 



PLANT PHYSIOLOGY AND ECOLOGY 



nutritive parenchyma. In addition to these three regions, common 

 to root, stem, and leaf, the root possesses a fourth, the root-cap, 

 which is pecuHar to it. The root-cap usually arises from the 

 dermatogen of the root-tip. It consists for the most part of paren- 

 chyma-like cells, which act as a cushion to protect the more delicate 



Fig. 11. The root system of the shade and sun forms of the false Solo- 

 mon's seal, Wagnera stellata. The shade form with few rootlets grows 

 at the edges of brooks where the holard is 30-60%; the sun fonn is a 

 gravel plant with a holard of 5-6%. 



meristem against tearing and crushing. As the root elongates, 

 the cells of the cap are extended along the surface, where they 

 gradually wear away or exfoliate. The same process occurs also 

 at the lower end of the cap, causing it to wear away as it grows. 



54. Detailed structure. A section of a typical root reveals 

 the three primary regions changed into epidermal, cortical, and 

 vascular tissues. The epidermal layer is merely the dermatogen 

 changed into a layer of permanent parenchyma-like cells. It 

 consists of epidermal cells, here and there drawn out into a long 



