288 



MOSSES AND FERNS 



CHAP. 



arrangement, both of the divisions of the leaf and the venation. 

 This is brought about in both cases by an unequal dichotomy, 

 by which one branch develops more strongly than the other, 

 so that the latter appears lateral. With the assumption of the 

 pinnate form the leaf also develops the wings or appendages 

 upon the axis between the pinnae. In the fully-developed leaves 

 of the mature sporophyte, the last trace of this is seen in the 

 ultimate branching of the veins, which is always dichotomous. 

 The second root arises close to the base of the second leaf, 

 and at first there seems to be one root formed at the base of 

 each of the young leaves ; in the older sporophyte the roots are 



Fig. i6o. — A, Longitudinal section; B, transverse section of roots from older sporo- 

 phyte of M. Douglasii, showing apparently more than one initial cell, X200. 



more numerous. Holle states that this is not the case in 

 Marattia, where only one root is formed for each leaf, in 

 Angiopteris two. This, however, requires confirmation in the 

 older plants. As the roots become larger it is no longer pos- 

 sible to distinguish certainly a single initial cell. The adjacent 

 segments themselves assume to some extent the function of 

 initials, and thus in place of the single definite apical cell a group 

 of apparently similar initials is formed, which takes its place 

 (Fig. 160). This seems to be in some degree associated with 

 -the increase in size of the roots.^ 



^ It is possible that a single initial may be present even here, but the 

 great similarity of the central group of cells makes this exceedingly difficult 

 to determine. 



