METABOLIC GRADIENTS 



73 



plant groups a more or less definite relation exists 

 between the directions o( the early divisions and the 

 major axis of the embryo. In these cases a more or less 

 distinct gradient in division rate, cell size, and cellular 

 constitution usually appears either at the beginning of 

 development or in early stages. On the one hand, this 

 gradient shows a definite relation to the position of the 

 Qgg with respect to surrounding parts of the parent or- 

 ganism, and, on the other, the region of smallest size and 

 most rapid division of the cells and most abundant and 

 deeply staining protoplasm is the region of highest rate 

 of reaction and becomes the apical region of the embryo. 

 Fig. 1 8 shows this gradient in the embryo of a moss, the 

 uppermost cell in the figure representing the apical region 

 of the embryo. 



In most of the higher plants only a portion of the egg 

 takes part in the formation of the embryo, the remain- 

 der forming a suspensor, a stalk on which the embryo 

 is carried. Fig. 19 shows the cellular gradient in the 

 early developmental stage known as the proembryo 

 of Ginkgo, a gymnosperm related to the conifers. The 

 embryo proper arises later from the small-celled tissue 

 in the lower part of the developing egg. Some of the 

 cycads also show a very definite gradient of this sort. 

 In the angiosperms, the higher seed plants, where the 

 egg is attached to the wall of the embryo-sac, the embryo 

 arises from its free apical end. 



A characteristic feature of the plant individual in all 

 except the simplest forms is the growing or vegetative 

 tip. This growing tip is the region of most active 

 nuclear division and growth and with rare exceptions 

 forms the free end of the individual and gives rise to 



