472 MOSSES AND FERNS CHAP. 



of tracheids of the older bundle. The intermediate cells divide 

 much more rapidly and constitute the phloem. The number 

 of endodermal cells in a cross-section corresponds generally to 

 the number of xylem and phloem masses. The peripheral 

 groups of tracheae early develop spiral thickenings upon their 

 walls, and sometimes there is but a single row of tracheae in each 

 xylem mass. Each of the three phloem masses of E. varie go- 

 turn has three narrow sieve-tubes in contact with the inner endo- 

 dermis surrounded by thin-walled cambiform cells. The thick- 

 enings upon the walls of the large central vessel form only at a 

 late period. 



Intercellular spaces arise at the angles of the outer endo- 

 dermal cell, and similar ones also between the outer cells of the 

 cortex, which becomes very spongy in the older roots. Numer- 

 ous brown root-hairs, like those upon the rhizome, cover the 

 surface of the root. A pericycle is quite absent, and the sec- 

 ondary roots arise from the inner endodermis in direct contact 

 with the tracheids. The latter, as will be seen from the figure, 

 lie between two endodermal cells, and the young root lies there- 

 fore not directly opposite, but to one side of the corresponding 

 xylem mass. The young roots may arise from either of these 

 endodermal cells, and consequently there is formed a double 

 row of rootlets corresponding to each xylem mass of the 

 bundle. Shortly after the rootlet is formed, the endodermal 

 cell outside it divides by a tangential wall, and this develops into 

 a double layer of cells completely enclosing the young rootlet 

 (Van Tieghem (5), p. 395). A similar "digestive pouch" is 

 formed, according to Van Tieghem, in the roots of many Ferns, 

 but is in these derived from the cortex outside the endodermis. 

 The double endodermis of the bundle of the older root shows 

 the characteristic foldings of the radial walls only upon the outer 

 cells. 



Cormack ( i ) has recently published a paper showing that in 

 E. maximum (telmateia) there is a slight secondary increase in 

 thickness in the nodes of the stem, due to the presence of a 

 genuine cambium, not unlike that in the stem of Botrychium. 



The Sporangium (Bower (15)) 



In all species of Equisetum the sporangia are formed upon 

 the under side of peltate sporophylls arranged in closely-set 



