THE VASCULAR CRYPTOGAMS 55 



development of the rootlet, beyond forming a pedicel 

 by which it is connected with the vascular tissues of the 

 main root. 



The origin of the adventitious roots, which play so 

 important a part in the organisation of Ferns, follows the 

 same rule which holds good for the rootlets. Every ad- 

 ventitious root arises from an endodermal cell bordering 

 on one of the steles of the stem. In Fig. 26, for example, 

 the shaded cell marked r is destined to produce a root. 

 We see from this that the first differentiation of the 

 root-forming cells in the stem takes place very early. 



7. The Leaf 



The development of the leaf in Ferns, like that of the 

 stem and root, goes on at the apex, whereas in most flower- 

 ing plants the growth of the leaf chiefly takes place at 

 the base. It is only when still very young that a Fern 

 leaf grows by means of a single apical cell. This cell 

 soon divides up so as to form a row of marginal cells, all 

 of which take equal parts in the subsequent cell-formation. 



B. EEPRODUCTIVE ORGANS OF THE SPOEOPHYTE 



We have already seen that the sporangia of the Male 

 Fern are grouped in sori, and that the sori are seated on 

 the back of the leaf, over the lateral veins of a segment 

 or pinnule (see Fig. 18, B). 



Beneath each sorus is a prominent mass of tissue, 

 which we may call the placenta (see Fig. 28, r). This 

 receives a short branch from the vascular bundle immedi- 

 ately below it. The placenta grows out at its summit 

 into the kidney-shaped indusium, which consists of a 

 membrane, one cell in thickness, attached to the placenta 



