264 



MOSSES AND FERNS 



CHAP. 



A cross-section of the petiole of the earliest leaves of the 

 young plant shows but a single nearly central vascular bundle, 

 but as the plant grows older the number becomes much larger, 

 and may reach ten (Luerssen (8), p. 58). In leaves of mod- 

 erate size there are usually about four, and these are arranged 

 symmetrically. The ground tissue is composed mainly of 

 large thin-walled parenchyma and a well-marked epidermis. 

 The fibrovascular bundles are arranged in two groups, right and 

 left, and where there are four of them the inner ones are the 



O^ 



Fig. 144. — A, Part of a cross-section of the stem bundle of B. Virginianum, X200, — 

 lettering as in Fig. 142; B, a portion of the tracheary tissue, showing the peculiarly 

 pitted walls, X400. 



larger, and in cross-section crescent-shaped. The xylem occu- 

 pies the middle of the section, and is completely surrounded by 

 the phloem, i.e., the bundle is concentric, like that of the true 

 Ferns. In B. lunaria the bundle has the phloem only perfectly 

 developed on its outer side and approaches the collateral form. 

 B. ternatum and B. lunaria, while having concentric bundles, 

 also have the phloem more strongly developed on the outer side. 

 The tracheary tissue is much like that of the stem, but the 

 tracheids are smaller and the walls thinner. The smaller tra- 

 cheids show reticulate markings. 



