294 



STRUCTURE OF FERN-STEM 



section of the stem \'aries considerably in different Ferns. Some 

 {e.g. Gleichenia, the Bracken of the Tropics) possess but a single 

 central stele of comparatively large size, whilst many of the 

 Tree Ferns exhibit a complex system of concentric rings of steles. 

 In the Male Fern {Nephrodiiiin filix-nias) , again, the strands form 

 but a single ring (Fig. 162, B, .Si.). The arrangement and histo- 

 logical structure of the vascular tissues within each stele is, 

 however, essentiallj.' the same in all cases. The 3''oung Fern 



Ftg. 162. — Ncplirodiiiin filix-uias. A, Skeleton of vascular system showing 

 the cylindrical network of steles, with numerous leaf-trace strands 

 arising from the edges of the " leaf-gaps " (original). B, Transverse 

 section of the rhizome (after De Bary). l.h., leaf-bases ; /./., leaf- 

 trace strands ; Scl., sclerenchyma ; St., stcm-stcles. 



seedling always possesses but a single central strand which, as 

 the plants become older, enlarges and, in forms like Ncphrodinm 

 and Picris, gradually breaks up into the separate steles of the 

 adult. 



The relations 1)etween the \'ascular system of stem and leaf 

 are particularly plain in the Male Fern [Ncplwodiuni) . and are 

 easily decii)hered, if its rhizome be allowed to rot, so that all 

 but the hard vascular tissues decay. In the resulting skeleton 

 (Fig. 1(12, .\) the steles form a cylindrical network, in which each 



