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 w T hich 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 in shape, size, 

 and structure. Both are in the resting condition, and have a nucleolus 

 (Fig. 228). The nuclear membrane then disappears at the place of contact, 

 their cavities become one, the chromatin-reticulum of the one unites with that 

 of the other, and the resulting fusion-nucleus can scarcely be distinguished 

 from the nucleus of an unfertilised egg. Finally the nucleoli fuse also. The 

 details of the fusion of the second male nucleus with the central fusion-nucleus 



Fig. 228. 



Behaviour of the male and female nuclei of 

 Lily in fertilisation. (Mottier.) A, vermiform 

 male nucleus applied to the egg-nucleus (Lilium 

 Martagon). B, egg-cell of Lilium candidum 

 showing sexual nuclei in act of fusing. The 

 nuclear membranes have disappeared at the 

 place of contact. 



