l6 MINNESOTA BOTANICAL STUDIES. 



the embryo-sac. Tlie tissues immediately adjacent to the micropyle 

 do not grow so rapidly as those somewhat more remote, and so 

 the inner opening of the micropyle comes to occupy the bottom 

 of a narrow, cup-like depression in the top of the ingrowth just 

 described. This depression is filled, in the mature seed, with endo- 

 sperm, and in this portion of the seed the embryo develops. (Text 

 figure 2). The ingrowth of the outer integument is accomplished 

 almost wholly by a great increase in the size of the interior cells 

 of the region, which increase from an average size of 40 mic. x 48 

 mic. to one of 60 mic. x 190 mic, the longest dimension lying parallel 

 to the micropylar canal. There is comparatively little cell division 

 except in the inner layer of the integument where cell division is 

 for a time quite active. 



In the ripe seed, the whole outer integument except the epidermis 

 becomes fleshy, the invagmated portion being more dry and spongy 

 than the outer parts. No structure derived from either integument 

 becomes firm and hard in the mature seed, the hard part of which 

 is composed entirely of endosperm. 



The inner integument is at its maximum development in thickness 

 at, or soon after, the flowering period. Its inner columnar layer 

 maintains its character for some time by frequent vertical ceil 

 divisions, but finally its cells become stretched and much flattened 

 by the rapidly growing embryo-sac. The cells in the layer directly 

 outside of the columnar layer begin to break down and collapse 

 within ten days of the flowering period. This change gradually 

 involves the outer layers, until in full grown seeds the inner integ- 

 ument consists of little but the much flattened cells of the once 

 columnar layer, backed by a broken down and collapsed mass of 

 cells, or at most by a single layer of much flattened living cells. 



These changes are most marked in the middle portions of the 

 ovule, where growth is most rapid. In the immediate vicinity of 

 the micropyle, and also in the antipodal region the inner integument 

 retains permanently very much the same structure which it has 

 throughout, at the time of flowering. 



Endosperm. 



The primary endosperm nucleus divides even before the perianth 

 of the flower falls, and further divisions follow rapidly, the nuclei 

 passing around in the thin parietal layer of cytoplasm so that they 



