THE FERNS 131 



and is dependent upon the gametophyte for a long period 

 -indeed in some of these the gametophyte remains 

 alive for months, or even years, after the sporophyte 

 has become quite capable of self-support. 



It is the development in the sporophyte of these ex- 

 ternal members- -stem, leaf, and root- -which at once 

 distinguishes the fern from the moss, and it is the pres- 

 ence of these which enables the sporophyte to become 

 independent of the gametophyte, which soon perishes. 

 It must be remembered, however, that the young sporo- 

 phyte in the ferns is also dependent for a longer or 

 shorter period upon the gametophyte, just as is the case 

 permanently in the mosses, and the cases known among 

 the former, where the existence of the gametophyte 

 does not necessarily end when the sporophyte has be- 

 come independent, recalls at once the normal condition 

 of things among the Bryophytes. 



Of the original quadrants into which the fern-embryo 

 divides, one, as we have seen, becomes the apex of the 

 future stem, and this cell may retain its identity, persist- 

 ing as the apical cell of the axis of the plant. Thus in 

 the gigantic tree-ferns, the single initial cell at the apex 

 of the stem is the direct descendant of one of the four 

 primary cells into which the embryo was first divided. 

 The growth of all of the other original members of the 

 embryo is limited, the cotyledon and primary root very 

 soon dying and giving place to others. 



The size which the sporophyte finally reaches varies 

 extremely. Thus in some of the tiny filmy-ferns (Fig. 

 35, C) the delicate stem is hardly thicker than a hair, 

 and the fully developed leaves less than a centimetre 

 in length; on the other hand, some of the giant tree- 



