130 EVOLUTION OF PLANTS 
from the egg, agree exactly with those in the liverwort. 
embryo, and the great similarity in the structure of the 
young sporophyte in Bryophytes and Pteridophytes 
(Fig. 33, D, E) is one of the strongest evidences of the 
intimate relationship of the two great divisions of the 
Archegoniate. The young embryo consists at first of 
four nearly equal cells, arranged like the quadrants of a 
sphere, and in the lower ferns the young sporophyte 
retains this globular or oval form for a considerable 
time, and closely resembles the corresponding stages in 
certain low liverworts, e.g. Riccia. In the common 
ferns, however, there very early appears a marked devi- 
ation from the type found in the mosses. This is the 
indication of external members, absent in the embryo of 
the latter. Usually each of the four original quadrants 
of the young embryo becomes the starting-point for a 
special organ, and soon these are evident as the rudi- 
ments of the primary leaf or cotyledon, the stem or 
axis of the young sporophyte, the primary root, and the 
foot (Fig. 88, F). Each of these organs in the more 
specialized ferns shows a definite apical cell, and this 
apical growth in each of the members soon causes the 
young sporophyte to assume the character of an inde- 
pendent plant, the young fern, in short. The root 
elongates rapidly and soon fastens the young sporo- 
phyte to the earth, and as soon as the primary leaf is 
expanded, the little fern is quite independent of the 
gametophyte with which it is still connected by means 
of the foot, through which it is nourished until its own 
primary members are fully developed (Fig. 32, D). 
In the more generalized and lower ferns, the sporo- 
phyte retains much longer its undifferentiated character, 
