. Botany of the living plant. Botany; Plants. POLLINATION AND FERTILISATION 307 The passage of the pollen-tube direct to the micropyle is the usual, and probably the primitive course. Fertilisation in that way is called porogamy. But in a considerable number of plants it takes a course through the superficial tissues of the ovule. Sometimes it passes through the funiculus to the chalazal end of the embryo-sac, as in the Walnut and Casuarina : this is called chalazogamy. Sometimes an irregular course may be pursued, by traversing the integuments, as in the Elm. But here the course appears to be
. Botany of the living plant. Botany; Plants. POLLINATION AND FERTILISATION 307 The passage of the pollen-tube direct to the micropyle is the usual, and probably the primitive course. Fertilisation in that way is called porogamy. But in a considerable number of plants it takes a course through the superficial tissues of the ovule. Sometimes it passes through the funiculus to the chalazal end of the embryo-sac, as in the Walnut and Casuarina : this is called chalazogamy. Sometimes an irregular course may be pursued, by traversing the integuments, as in the Elm. But here the course appears to be very inconstant. It is doubtful whether these irregularities have any special significance, but it is worthy of remark that they occur in relatively primitive Families of Flowering Plants. The pollen-tube on entering the micropyle conveys with it the two male gametes enclosed in the cytoplasm of the tube (Fig. 226, A). Probably the turgor of the contents has its effect in rupturing the soft tip of the tube, and extruding its contents. The nuclei of the two gametes can shortly afterwards be recognised in the embryo-sac. The one passes into the ovum and fuses with its nucleus. The result of fusion of the male and female gametes is the zygote. The other passes on to the central fusion- nucleus and coalesces with it (Fig. 228). The mechanism of the movements within the embryo-sac is uncertain. It has been suggested that protoplasmic streaming may assist it. On the other hand, the peculiar form wThich the male nuclei sometimes take suggests indepen- dent movements, like those of the sperms of lower plants to which they correspond functionally (Fig. 227, B). Meantime the synergids shrivel, and begin to disorganise. Clearly their function is com- pleted on fertilisation. In the case of Lilium the more or less spiral form of the male nucleus, when it penetrates the ovum, has been seen to be retained till it is applied to the nucleus of the ovum. But the nuclei gradually become alike i
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