EMBRYOGENESIS IN GYMNOSPERMS 



183 



organising capacity of the distal embryo apex may be discerned. 

 Chamberlain records that the living suspcnsor in Ceratozamia can be 

 pulled out to a length of some 7-8 cm, 



Polyembryony within a single archegonium has also been observed, 

 e.g. in Macrozamia reidlei, there may be one to three additional lateral 

 embryos. These arise like the main embryo, each owing its formation 

 to a small group of actively dividing cells at the basal periphery of the 



Fig. 40. Dioon edule 



Embryo in longitudinal section, showing an early stage in the formation of cotyle- 

 dons. The enlarged basal region is the coleorhiza. The shoot and root apical 

 meristems are not yet clearly differentiated (after Chamberlain). 



proembryo. The primary suspcnsor may also fork and give rise to 

 equal embryos. A similar branching of the suspcnsor has also been 

 seen in Encephalartos. 



The Embryo. Contemporaneously with these suspcnsor develop- 

 ments, the embryonic region has been growing slowly. The differentia- 

 tion of the rather uniform mass of meristematic cells takes place very 

 gradually, but two cotyledons are eventually formed, while below them, 

 in the bulky basal region known as the coleorhiza, an endogenous root 

 has its inception, Fig. 40. In the early stages of this embryonic 

 development, when the whole of the apical region is still growing, both 

 periclinal and anticlinal divisions take place in the cells of the outermost 

 tissue. The distal, superficial layer, which by virtue of its position 

 becomes the dermatogen, appears to assume a haustorial function, 

 Fig. 40, but whether the main nutrition of the embryo is by way of this 

 tissue region, or acropetally by way of the suspcnsor, is still a matter 

 for conjecture. After the formation of the cotyledons, the sunken 

 shoot apex gives rise to the first leaf and to one or more associated 



