MINNESOTA BOTANICAL STUDIES. 



Chalazal Vesicle. 



After the flowering period the antipodal end of the embryo-sac 

 increases but little in size, but cells of the lower part of this region 

 become vesicular, and grow down into the adjacent chalazal region, 

 breaking down the large cells which lie between the embryo-sac and 

 the vascular tissue of the ovule. A large cavity is thus formed in 

 the chalazal region of the ovule which is lined by a very thin layer 

 of protoplasm containing a few very large nuclei (Text figure 2, A 

 and B). This cavity is cut off from the cavity of the embryo-sac 

 by thin cell w^alls across the narrow part of the embryo-sac (Fig. 

 ID, PI. IV). 



The origin of the invading cells could not be determined with 

 entire certainty, but it appears most probable that they are special- 

 ized endosperm cells which become cut off from the main body of 

 the endosperm very shortly after flowering. In sections taken at 

 flowering time the antipodal nuclei are seen as small darkly staining 

 bodies which appear to be in rapid degeneration. In preparations 

 made a little later in which the endosperm is actively developing, 

 three to five very large nuclei are seen occupying the narrow part 

 of the embryo-sac. They closely resemble the ordinary endosperm 

 nuclei, and it seems much more likely that this is their character 

 than that the antipodal cells, already in an apparently degenerate 

 conditions should suddenly become active again. The protoplasm 

 surrounding these nuclei becomes cut off into distinct cells long 

 before any general cell formation takes place in the endosperm, and 

 it is the growth of these cells down into the chalazal tissues which 

 results in the vesicular cavity just described. 



This structure undoubtedly serves to put the embryo-sac into 

 closer communication with the vascular tissue of the ovule, and 

 appears to continue functional throughout the ripening of the seed. 



The Embryo. 



After fecundation the oosperm forms a thin cell wall about itself. 

 The cell thus formed is at the extreme micropylar end of the em- 

 bryo-sac abutting on the cap-like remains of nucellus (Fig. 2, PI. 

 IV). It is covered with a thin layer of cytoplasm belonging to the 

 endosperm, while the degenerate remains of the synergids can some- 

 times be distinguished at one side of it. close to the base. The 

 oosperm cell which is at first almost spherical, increases gradually 



