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 

 Archegoniatse. 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. 33, 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, throuo-h which it is nourished until its own 



o 



primary members are fully developed (Fig. 32, D). 



In the more generalized and lower ferns, the sporo- 

 phyte retains much longer its undifferentiated character, 



