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- 
plied, to decide whether the endosperm is gametophytic (x) tissue or 
sporophytic (2x) tissue, it is found that it is at least 3* tissue. To call 
3* 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 completest 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. 
Perisperm. The storage region of some seeds is not the endosperm 
but the perisperm, which is the nucellar tissue surrounding the embryo 
