772 PART IV. THE PHYSIOLOGY OF PLANTS. 



parthenogenetically produced spores; and the phenomena of apogamy and 

 apospory in the higher plants also require investigation from this point of 



21. Sexual Reproduction. The sexual process consists 

 typically, though not exclusively (see pp. 225, 275) in the fusion of 

 two gametes, that is, of two sexual reproductive cells, and it is to 

 this typical case only that reference is here made. 



The process consists essential ly in the fusion of the homologous 

 parts of the gametes. In the Phanerogams, in which the process 

 has been most minutely investigated, it is briefly as follows : 

 The male gamete, being smaller and more active than the female, 

 enters the latter (see Figs. 298, 302) ; then the centrospheres of 

 the two gametes fuse, and then their nuclei, male nucleus (or 

 pronucleus) with female nucleus (or pronucleus), to form the 

 nucleus of the oospore, and with that the process is complete. 

 But the fusion of the nuclei is not accompanied by any fusion of 

 their respective chromosomes ; these remain distinct, so that there 

 are twice as many of them as in the nucleus of either gamete: 

 but when the oospore begins to germinate, the first division of its 

 nucleus takes place in such wise that each half receives an equal 

 number of chromosomes derived from each of the sexual nuclei. 

 No doubt the process is essentially the same in all other cases. 



The first question which naturally arises is as to the nature of 

 sexuality ; the question, namely, as to what difference, if any, can 

 be observed between a gamete and an asexually-produced spore. 

 To this question no answer can at present be given ; no difference 

 can be detected between a gamete and a spore; in all plants in 

 which the matter has been investigated, in a Fern, for instance, 

 it is now known that the number of chromosomes in the nucleus is 

 the same in the gamete as in the spore. 



It was thought that there was an essential observable difference between 

 a gamete and a spore, of this nature, that the nucleus of the gamete contained 

 fewer chromosomes than did that of the spore. It had been observed in many 

 cases that, as part of the development of a gamete, a portion of the proto- 

 plasm of the mother-cell was thrown off as a polar body (see p. 82), and it was 

 concluded that this involved the reduction by half in the number of chromo- 

 somes in the nucleus of the gamete, and thus caused it to differ materially 

 from a spore. But this view is no longer tenable in face of the fact that the 

 reduced number of chromosomes in the nucleus is characteristic, not on'y of 

 the gametes, but also of the whole gametophyte, and in fact of the spore itself 

 also (see p. 771). The polar bodies can now only be regarded as sister-cells of 

 the gametes and as equivalent to them, though not functional. 



