SPERMATOPHYTES 
269 
the embryo sac by way of the micropyle, but pierces directly through 
the region of the ovule beneath the embryo sac (chalaza) and enters 
the embryo sac from below. Among the more familiar plants in which 
chalazogamy has been found are 
the walnuts and elms. Entrance 
by the micropyle is called porog- 
amy; and there are other routes 
used by the pollen tubes of cer- 
tain plants, intermediate between 
true chalazogamy and porogamy. 
Fertilization. After the tip 
of the pollen tube has entered 
the sac, it enlarges very much, 
usually destroys one of the 
synergids, and finally discharges 
the two male cells or nuclei 
(fig. 599). One of the male 
cells passes to the egg and fer- 
tilization is accomplished. The 
other male cell passes deeper 
into the sac, comes into contact 
with the fusion nucleus, and fuses 
with it. Into the structure of the 
primary endosperm cell (or nu- 
cleus), therefore, three nuclei 
have entered : an antipodal 
, i i j FIG. 599. Fertilization in Silphium: 
polar, a micropylar polar, and a sy> undestroye d synergid; pt, swollen tip of 
male nucleus. This participa- pollen tube, still with some contents (#); 
tion of both male cells in nuclear **> coiled male cel1 in contact with e gg 
..... . nucleus (o); sp^, curved male cell in contact 
fusions in the same embryo sac with fusion nuc i eu s (*).- After LAND. 
has been called double fertiliza- 
tion, and it is perhaps the greatest puzzle connected with the embryo 
sac of angiosperms. 
Double fertilization. This phenomenon was first described in 1898, 
but subsequent investigation has indicated that it is probably of universal 
occurrence among angiosperms. It means that one male cell enters 
into the formation of the embryo, and the other into the formation of the 
endosperm. This raises a question as to the nature of the endosperm of 
angiosperms. The old view was that it is belated tissue of the female 
