THE EMBRYO 



317 



be delayed by an intervening period of dormancy. In the embryo of 

 some taxa, the four distal octants form tlie cotyledons and the stem 

 apex; the basal octants form the hypocotyl; the region of union of sus- 

 pensor and embryo proper forms the root apex. The root apex may be- 

 come distinct much later than the other parts of the embiyo and is 

 apparently not an appendage (Fig. 119, 19). 



After-ripening. The embryo is generally considered mature when the 

 seed is shed — mature in the sense of readiness to establish itself as a 

 seedling. But seeds are not "ripe," morphologically, until the embryo is 



Fig. 120. Early stages in ontogeny of embryo of Sagittaria guaijanensis. Enlarged 

 basal cell of filament forms prominent part of suspensor. O, mature embryo. (After 

 Johri. ) 



full-grown; at shedding, the seeds of many taxa are immature, and a 

 period of intraseminal growth must ensue before the embryo is ready 

 for germination. The existence of this concealed development has been 

 largely overlooked in the twentieth century; in the many studies of 

 dormancy, there has apparently been little or no correlation of stages of 

 embryo development with external conditions. The term after-ripening 

 has been used to cover changes necessary to germination — relations of 

 temperature, humidity, and light — but, in many taxa, changes in in- 

 ternal growth are at least as important as these physiological relations. 

 Continuous development of the embryo is well known in some 

 gymnosperms; it is characteristic of the cycads, Gnetum, and Ginkgo; 



