470 BOTANY OF THE LIVING PLANT 



one type and two of the other. Since the spores of these two types 

 germinate, and each gives rise to a gametophyte, and ultimately to 

 the gametes, it follows that the gametes produced from these two types 

 of spore w'ill not be all alike, but will share the same segregation of 

 cliaracters as the spores that gave rise to them. - 



A second point is that the reduction of chromosomes to one half 

 having taken place in the tetrad, all the succeeding products of their 

 germmation will be on the reduced or haploid footing, having only 

 X chromosomes. This applies to the whole gametophyte generation, 

 and to the gametes themselves, which are haploid. 



Fertilisation consists in the fusion of gametes. In many Algae 

 these are clearly primordial cells, with cytoplasm and nucleus. But 

 as the spermatozoids became more specialised in higher types, such 

 as the Mosses and Ferns, their cytoplasm was reduced to negligible 

 proportions. The opinion is widely held that the cytoplasm takes 

 no essential part in the fertilising process beyond acting as a carrier 

 for the male nucleus, or as a storage place for nutriment in the egg. 

 However this may be, it is certain that the nucleus makes up the 

 great proportion of bulk of the spermatozoid both in the higher 

 Animals and Plants, while the most prominent features in fertilisa- 

 tion arise from the male and female nuclei. The former passes into 

 the cytoplasm of the egg : it approaches, and applies itself to the 

 female nucleus. The nuclear membranes disappear along the surface 

 of contact. Both nuclei have been seen in favourable cases to enter 

 the so-called spireme state with disengaged chromosomes, as if about 

 to divide. The male and female chromosomes pair, and subsequently 

 each divides transversely. One half of each of the pairs moves to either 

 pole of the first nuclear spindle of the zygote, so that the male and 

 female elements are equally distributed. Both male and female 

 gametes being haploid when fertilisation takes place, the double 

 number of chromosomes is restored. The diploid sporophyte is thus 

 initiated, and the chromosome-cycle, — sporophyte, — gametophvte, — 

 sporophyte, — may be repeated indefinitely. 



Hybridisation. 



If the gametes involved in producing such a succession of genera- 

 tions were uniform throughout in their origin and in all their char- 

 acters, the organisms produced might be expected to remain constant. 

 But the parents that produce the gametes are not themselves exactly 

 alike, and the gametes produced from them will therefore differ. 

 Differences exist between individuals, strains, varieties, species. 



