EMBRYO, SEED AND CARPEL IN THE DATE. 



151 



walled but growing endosperm, b\ With a slight advance 

 in age, it becomes evident that the inner part of zone b is 

 giving cells to be digested, and these then become part of 

 zone a . At the same time, zone b' becomes narrowed down 

 by the advancing zone c (f. 16), endosperm in which re- 

 serve cellulose is being laid down. This results in crowding 

 the cells in which divisions are taking place into a narrower 

 compass, and they occupy a region whose position is approx- 

 imately represented by the narrow cross-hatched line between 

 b and b\ It will be observed that the actual histological 

 results of tension are seen at first in zone b while in later 

 stages they pass over into zone 6', as represented in the dia- 

 grams. That is to say, thin-walled endosperm cells entirely 

 disappear, but are represented by cells which are secondarily 

 transversely divided and at the same time somewhat second- 

 arily thickened. Zone c disappears because tannin does so. 

 This fact is represented in the diagrams by the line of de- 

 markation overtaking, and becoming coincident with, the 



inner limit of zone c . 



In endeavoring to understand clearly the course of events 



it is necessary to keep in mind the directions of growth (1) 

 of the endospermic plug tissue overlying the base of the 

 embryo, and which thickens independently of the main mass 

 of endosperm; (2) of the embryo, which is growing in thick- 

 ness and length, on the one hand; and (3) of the main body 

 of endosperm, expanding radially in all directions toward 

 the inner integument as a limit. The enlarging endosperm 

 cells, namely those in the interior, push against the dividing 

 endosperm cells, which are peripherally placed. These oc- 

 cupy a position, therefore, between the inner integument 

 and the inner part of the endosperm, the cells of which 

 are enlorigating radially. These thin walled, dividing cells 

 constitute a tension zone. But in the region of the embryo, 

 the digestive zone takes the place of the inner integument, 

 so that the tension zone, where the secondary division there- 



■ 



fore are taking place, dips inwardly. The plug tissue how- 

 ever is also in division, and this independent disc of ten- 

 sion tissue meets and merges with the other at the place 





i 



