174 MORPHOLOGY OF ANGIOSPERMS 



nuclei that apparently fuse into one. The irregular and usu- 

 ally large number of chromosomes found in the nuclei of endo- 

 sperm tissue is doubtless due to " double fertilization v and 

 other nuclear fusions. 



The parietal plate of cells by division gradually encroaches 

 upon the general cavity of the embryo-sac, either filling it up 

 compactly about the embryo, or leaving more or less of a cavity 

 containing cell sap, which in the coconut becomes of extraordi- 

 nary size. 



In many cases a fully developed endosperm is more or less 

 displaced by the growing embryo, so that in the mature seed it 

 may be much reduced or even obliterated. Among the Mono- 

 cotyledons the embryo of the Gramineae is at first completely 

 invested by endosperm, but becomes eccentric by displacing it 

 on one side; and the embryo in some Araceae finally replaces 

 all the endosperm ; but for the most part the Monocotyledons 

 are characterized by retaining the endosperm in the mature 

 seed. Among the Dicotyledons, however, it is characteristic of 

 certain families, among the important ones being Cupuliferae, 

 Leguminosae, Cucurbitaceae, and Compositae, for the embryo 

 to have entirely displaced the endosperm at the maturity of the 

 seed, the gain in size being almost entirely in the cotyledons. 

 It must not be supposed that in all cases the formation of endo- 

 sperm continues from the first free nuclear division to a tissue 

 filling the embryo-sac. Illustrations could be introduced show- 

 ing a cessation of endosperm formation at every stage. It may 

 stop with a few free nuclei, or with the parietal placing of free 

 nuclei, or with a parietal plate of tissue. An interesting case 

 is that of Tricyrtis (Liliaceae), recently described by Ikeda, 54 

 in which free endosperm nuclei are distributed through a sac 

 full of cytoplasm, and assume very irregular and bizarre forms, 

 the parietal position never being assumed. 



The second general method of endosperm formation — 

 namely, that in which the first division of the primary endo- 

 sperm nucleus is accompanied by a wall dividing the sac into 

 two chambers — is found chiefly among Dicotyledons, and among 

 them it is especially characteristic of saprophytic and parasitic 

 forms, Cuscuta being a marked exception in that its endosperm 

 begins with free nuclear division. Usually the wall divides the 

 sac into two approximately equal chambers, but naturally the 





