. A textbook of botany for colleges and universities ... Botany. 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 ver
. A textbook of botany for colleges and universities ... Botany. 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. S99). 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 polar, a micropylar polar, and a male nucleus. This participa- tion of both male cells in nuclear fusions in the same embryo sac 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. Silphium : Fig. 599. — Fertilization sy, undestroyed synergid; pt, swollen tip of pollen tube, still with some contents (»); spi, coiled male cell in contact with egg nucleus (o); sp2, curved male cell in contact with fusion nucleus (c). — After Please note that these images are extracted
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