270 MORPHOLOGY 



gametophyte, which in some way is stimulated to develop by the polar 

 fusion. But when the part played by the male cell was discovered, it 

 was suggested that this triple fusion is a real fertilization, which would 

 > mean that the so-called endosperm is a sporophyte, the twin of the em- 

 bryo. If this is true, the endosperm of gymnosperms is not the same as 

 that of angiosperms. If the test of the number of chromosomes be ap- 

 pUed, to decide whether the endosperm is gametophytic {x) tissue or 

 sporophytic {2x) tissue, it is found that it is at least ^x tissue. To call 

 yc tissue gametophytic seems to make the test of little value. If the 

 triple fusion be analyzed, it will be noticed that one cell is the micropy- 

 lar polar, which is sister to the egg, and another is a male cell. If only 

 these two cells fused, it could hardly fail to be regarded as fertilization ; 

 but the third cell that enters into the fusion is a vegetative cell (or nucleus) 

 "^from the antipodal end of the sac, so that the real nature of the fusion 

 is confused. Perhaps it would be better to speak of the endosperm of 

 gymnosperms as female gametophyte, and to reserve the name endosperm 

 for this problematical tissue in the embryo sac of angiosperms. 



Endosperm 



Development. — As described above, the endosperm of angiosperms 

 is produced by the triple fusion nucleus. It usually begins with free 

 nuclear division, but sometimes it begins with wall formation that 

 chambers the sac. In its completes! development it forms a tissue that 

 fills the embryo sac and is packed about the embryo. In some groups 

 the endosperm may develop only as a few free nuclei, so that it may be 

 regarded as suppressed, as in Helobiales and orchids (groups belonging 

 to monocotyledons). There is also great variation in the permanency 

 of endosperm which has been fully developed. It may be used up by 

 the embryo during the ripening of the seed, as in peas and beans (Legu- 

 minosae) ; or it may persist in the mature seed, being used up by the 

 embryo during germination, as in the cereals. In structure, permanent 

 endosperm tissue has no intercellular spaces, and the cell wall may be 

 thin or thick, an excessive thickening occurring in bony seeds, notably 

 in the date and in the so-called vegetable ivory, both from the seeds of 

 palms. Sometimes by its continued growth the endosperm has been 

 observed to burst the seed coats, turn green, and form intercellular 

 spaces. 



Perispenn. — -The storage region of some seeds is not the endosperm 

 b\U the perisperm, which is the nucellar tissue surrounding the embryo 



