246 



MOSSES AND FERNS 



CHAP. 



near the periphery, and have in consequence the same regular 

 arrangement seen in similar cells of the higher plants. 



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

 young plant show but a single nearly central vascular bundle, 

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

 and may reach ten.^ In leaves of moderate 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 



n • 



n 





-Ph.. 





^ 



-Xy 



vn\Q\ 



Fig. 126.— Part of a vascular bundle from the petiole of B. Virginianum, X245; xy, xylem ; //;, 

 phloem ; s,s, sieve-tubes ; B, two sieve-tubes in longitudinal section, X 490 ; sp, sieve-plates ; n, nuclei. 



bundles are arranged in two groups, right and left, and where 

 there are four of them the inner ones are the larger, and in 

 cross -section crescent -shaped. The xylem here occupies 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. ternatmn and B. lunaria, while having concentric bundles, 



Luerssen (8), p. 58S. 



