450 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. variegatuni 

 has three narrow sieve-tubes in contact with the inner endodermis 

 surrounded by thin-v\^alled cambiform cells. The thickenings 

 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. 

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

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

 secondary roots arise from the inner endodermis in direct con- 

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

 figure, lie between two endodermal cells, and the young root 

 lies therefore 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.-^ 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 char- 

 acteristic foldings of the radial walls only upon the outer cells. 



Cormack ^ has recently published a paper showing that in 

 E. maximum {telmateid) 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. 



TJie Sporangia 



In all species of Equisetum the sporangia are formed upon 

 the under side of peltate sporophylls arranged in closely -set 



1 Van Tieghem (5), p. 395. ^ Cormack (i). 



