THE VASCULAR CRYPTOGAMS 



91 



ex. 



en* 



the stem, which are much larger (see Fig. 38). All 

 the roots seen on a mature plant are adventitious ; 

 the main root of the embryo only lasts a short time ; its 

 structure is like that of the adventitious roots, and our 

 Fig. 41, which was drawn from the main root, will serve 

 to represent either. The young parts of the root bear 

 numerous root-hairs. They have a wide cortex, enclosing 

 a small and 

 simple central 

 cylinder, the 

 structure of which 

 is usually either 

 triarch ortetrarch. 

 The arrange- 

 ment of the xy lem- 

 and phloem- 

 groups is that 

 usual in roots ; 

 the centre is 

 occupied by a 

 large tracheide. 

 The chief peculiar- 

 ity of the root 



FIG. 41. Equisetum; transverse section of 

 main root, x, triarch xylem ; ph, phloem 

 (three groups) ; en, double endodermis ; ex, 

 thick - walled exodermis ; ep, epidermis. 

 Magnified about 100 diameters. (After 

 Buchtien. ) 



is its double 



endodermis ; the inner layer taking the place of a peri- 

 cycle, which is quite absent. That this layer is really 

 part of the endodermis is proved by the development, 

 and by the fact that its cells fit on accurately to those 

 of the outer sheath, which alone has the usual endo- 

 dermal structure (see Fig. 41, en). This double 

 endodermis is a character quite peculiar to the roots 

 of Equisetum. The origin and mode of growth of the 

 root will be considered in the next section. Apart 



