LIFE HISTORY OF PINUS 87 



of cells, three to five cells in thickness, which are more or less 

 clearly delimited from the surrounding tissue by their slightly 

 larger nuclei, their somewhat radial arrangement about the 

 macrospore-mother-cell as a center, and, in some instances, by 

 a rather indefinite and broken space which separates this group 

 of centrally lying cells from the adjacent nucellar tissue (fig. 

 124, plate XII). At the close of the tetrad-division these cells 

 have become much more conspicuous by the increase in the size 

 of their nuclei, the somewhat greater density of their cytoplasm, 

 and by the presence just exterior to them of an interrupted layer 

 of tabular cells which are evidently undergoing disintegration. 

 The disintegrating cells usually appear on one side first then at 

 other points about equally distant from the young gametophyte 

 (figs. 66, 69, plate VI ; 124, plate XII, 148, and plate XIV). It 

 was to this tissue, immediately surrounding the young endo- 

 sperm, together with the disintegrating cells just exterior to 

 it, that Strasburger gave the name " spongy " tissue, and for 

 convenience I shall use this term in speaking of it. 



Ovules are frequently found during the summer and fall 

 which, so far as external appearances go, are perfectly normal, 

 but, when prepared for study, reveal the fact that either the 

 macrospore-mother-cell has never divided or the macrospore, if 

 formed, has not developed. Such ovules do not renew their 

 growth in the following spring. In those cases in which the 

 development of the mother-cell or of the young gametophyte 

 is arrested, very characteristic changes occur in the spongy 

 tissue. These cells grow and become rich in cytoplasm even 

 when the mother-cell does not divide, or when the macrospore 

 fails to germinate. But after a time they, too, become inactive, 

 their cytoplasm is gradually lost, their nuclei become dense 

 and deeply staining, and their cell-walls are very greatly thick- 

 ened (fig. 163, plate XV). This state of disintegration may 

 enter in at any time during the first period of growth but it is 

 more common before any divisions have occurred in the macro- 

 spore. When the mother-cell fails to divide, the cells of the 

 spongy tissue may grow until they almost equal it in size before 

 showing signs of breaking down. In such instances they bear 

 a very striking resemblance to the mother-cell, and might easily 



