Z2>2 



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 ( (ii), 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. 



par 



m 



Fig. 184. — Woodwardia radicans. A, Tracheids, t, and wood-parenchyma, par., fron? 

 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 pe datum) 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 



