332 



MOSSES AND FERNS 



CHAP. 



parenchyma, and in the phloem the sieve-tubes are accompanied 

 by bast parenchyma. 



Outside the phloem is a layer of cells, which may be double 

 in some places, and which usually contain a good deal of starch. 

 According to Strasburger ((n), Vol. 3, p. 446) these cells do 

 not constitute a true pericycle, but belong to the cortex. They 

 are sister-cells of the endodermis, which is thus, not the inner- 

 most cortical layer, but the next but one. The endodermal cells 

 show the characteristic thickenings on their radial walls. 



D. 



FIG. 184. Woodwardia radicans. A, Tracheids, t, and wood-parenchyma, par., from 

 the rhizome, X225 (about); B, longitudinal section of two tracheids, more strong- 

 ly magnified; C, section of the wall between two tracheids; D-F, sieve tubes. 



The Leaf 



While the leaf in a few of the Leptosporangiatse is simple, 

 in much the larger number it is compound, either dichotomously 

 branched (Adiantum pedatwn) or more commonly pinnately 

 divided. Owing to the great irregularity of the divisions and 

 slow formation of new segments in the stem apex, it is exceed- 

 ingly difficult to determine positively whether each segment of 

 the stem apex produces a leaf, but this seems probable. The 

 leaf appears as a blunt conical emergence, whose apex is occu- 

 pied by a single large apical cell, which in nearly all forms 

 examined is wedge-shaped and forms two rows of segments. 

 As the leaf grows it assumes the form of a flattened cone with a 



