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- 
