268 MOSSES AND FERNS chap. 



clear separation of the root-cap from the body of the root that 

 is so distinct in Equisetum, for example. 



The central cylinder of the root is bounded by an endoder- 

 mis whose limits, however, are not so clearly defined as in the 

 more specialised Ferns. The number of xylem and phloem 

 masses varies, even in the same species. In B. Virginianum 

 the larger roots show three or four xylem masses (Fig. 147). 

 B. ternatum^ has usually a triarch bundle, while B. lunaria is 

 commonly diarch (Holle (i), p. 245). The elements both of 

 the xylem and phloem are much like those in the stem and do 

 not need any special description. The roots increase consider- 

 ably in diameter as they grow older, but this enlargement does 

 not take place at the base, where the root is noticeably con- 

 stricted. The enlargement is due entirely to the cortical tissue, 

 and is mainly simply an enlargement of the cells. The diameter 

 of the central cylinder remains the same after it is once formed. 

 In the outer part of the root, as in the stem, there is a develop- 

 ment of cork. 



The Sporangium 



In the simplest forms of B. simplex the sporangia, which 

 are much larger than those of 5. Virginianum, form two rows 

 very much as in Ophioglossum; but in all the more complicated 

 forms the sporangiophore branches in much the same way as 

 the sterile part of the leaf, and the ultimate segments become 

 the sporangia. In B. Virginianum the development of the 

 individual sporangia begins just about a year previous to their 

 ripening, and if the plants are taken up about the time the 

 spores are shed, the earliest stages may be found. The sporan- 

 giophore is at this time thrice pinnate in the larger specimens, 

 and an examination of its ultimate divisions will show the 

 youngest recognisable sporangia. These form slight elevations 

 growing smaller toward the end of the segment (Fig. 148), 

 and exact median sections show that at the apex of the broadly 

 conical prominence which is the first stage of the young sporan- 

 giumi there is a large pyramidal cell with a truncate apex. 

 Holtzman (i) thinks the sporangium may be traceable to a 

 single cell, and that the divisions at first are like those in a 

 three-sided apical cell. I was unable to satisfy myself on this 



* B. ternatum = B. obliquum (Underwood (5) p. 72). 



