xo THE STRUCTURE OF ECONOMIC PLANTS 



also serve to maintain pressure relations in the stele and prevent the 

 accumulation of air in the conductive tissues. Schwendener (37) 

 has proposed that the primary function is that of mechanical sup- 

 port and protection to the stele when the outer cortical cells are 

 lost. The presence of abundant quantities of starch in the endo- 

 dermis of the stem in some plants suggests that storage may be an 

 important function. In Linum, Cucurbita, and others, the endo- 

 dermal layer may be involved in the initial stages of lateral root 

 development, forming a part of the root cap. 



The endodermis is a relatively restricted tissue constituting 

 the inner limit of the cortex and lying adjacent to the stele. It 



Fig. 7. Transection of a young primary root of potato showing Casparian thickenings 

 on the endodermal walls: co, cortex; en, endodermis; mx, metaxylem; pel, pericycle; ph i, 

 primary phloem; px, protoxylem. 



is usually a single layer in thickness, but an exception has long 

 been known in the root of Brassica oleracea, reported by Woronin 

 (46), in which a double endodermis may develop. Other excep- 

 tions occur in Lactuca, in which single cells or sectors of the 

 endodermis may undergo tangential division; and in Cynara Scoly- 

 mus, where it becomes two-layered early in ontogeny. 



The endodermal cells form a compact layer which is uninterrupted 

 by intercellular spaces. In transection, they are four-sided, oval, 

 or elliptical, and more or less extended in the tangential direction; 

 while, in the vertical plane, they are elongated with end walls that 

 are usually transverse. The cells retain their protoplasts and 

 possess all the potentialities of typical parenchyma, including the 

 capacity for cell growth and division. The nucleus is prominent, 

 and the cytoplasm frequently contains starch grains in such large 



