CHAPTER IV. SPECIAL PHYSIOLOGY OF REPRODUCTION. 771 



individual which, in all plants presenting alternation of genera- 

 tions, is the sporophyte. 



In some cases the spore is formed from a gamete without any sexual 

 process, that is, parthenogeneticall'j (p. 87) ; this applies to the imperfectly 

 sexual gametes of Ulothrix and Ectocarpus mentioned above, whence it follows 

 that there can be both male and female parthenogenesis ; to the azygospores 

 of some Zygomycetes (pp. 288, 290) ; to the oospores developed in the oogonia 

 of the Saproleguiaceae (p. 291), and to that of Chara crinita (p. 254). 



It has been ascertained, in plants other than the Thallophyta, 

 that the nucleus of the sexually-produced spore contains twice as 

 many chromosomes (p. 119) as does that of either of the gametes ; 

 for instance, if the number of chromosomes in the nucleus of the 

 gamete be twelve, that in the nucleus of the oospore will be 

 twenty-four; and since the sporophyte is developed from the 

 oospore, the number of chromosomes in the nucleus of each of its 

 cells is also twenty- four, and is twice as great as that in the 

 nucleus of the cells of the gametophyte. Bat, since the gameto- 

 phyte is agamogenic, being developed from the asexually-produced 

 spores of the sporophyte, the question arises as to how this 

 reduced number of chromosomes is arrived at. It is simply due 

 to the fact that when the nucleus of a spore-mother-cell is about 

 to divide, the fibrillar network breaks up into only half the 

 number of chromosomes characteristic of the sporophyte (for 

 instance, twelve instead of twenty-four) ; hence the spore, when 

 formed, contains the reduced number of chromosomes character- 

 istic of the gametophyte and is, in fact, the first term of that 

 generation. 



These points have not yet been fully investigated in the Thallophyta, in 

 which group various special questions arise. For instance, as already pointed 

 out, in many of these plants the gametophyte is directly developed from the 

 sexually-produced spore, and yet there can be little doubt that the nuclei of 

 the gametophyte each contain only half the number of chromosomes present 

 in the nucleus of this spore; for, were it otherwise, each sexual act would 

 involve the doubling of the number of the chromosomes in the nuclei of the 

 succeeding generation, and by continued repetition of the process the number 

 would become indefinitely large. It seems probable that a reduction in the 

 number of the chromosomes takes place early in the germination of the 

 sexually-produced spore. Thus it has been observed in certain Desmids that 

 the nucleus of the germinating zygospore divides into four, but the product is 

 only two new individuals each with a single nucleus ; and similarly, that the 

 nucleus of the zygospore of Spirogyra divides into four, yet the unicellular 

 embryo contains but a single nucleus. Then there is the further question as 

 to the number of chromosomes in the nuclei of the gonidia and in those of 



