Nov. 1894.] BOTANICAL SOCIETY OF EDINBUEGH. 277 



merely on external form, but even on the minute structure 

 of the nuclei of the component cells. 



This new aspect of the question of alternation was first 

 opened up by Overton. It had already been ascertained, 

 on ground of the observations of Guignard and of Stras- 

 burger, that in Angiosperms the reproductive cells contain 

 only half as many chromosomes as the vegetative cells. 

 Overton suggested a widening of the thesis : " that the 

 reduced number of chromosomes in the nucleus is a feature 

 which is peculiar, not to reproductive cells, but to the 

 whole sexual generation." This has now been shown to 

 be the case for various Gymnosperms, by Overton, and by 

 Strasburger and Dixon ; while Guignard, Strasburger, and 

 Overton have shown it also for Angiosperms. But practical 

 difficulties have presented themselves in the solution of the 

 question in the Archegoniatae, though Overton concludes 

 that such results as were obtainable by him are favourable 

 to the hypothesis that the reduction takes place in the 

 spore-mother-cells, and persists throughout the gameto- 

 phyte. Observations of Humphrey and of Strasburger on 

 Osnninda have since supported this conclusion for the 

 Pteridophyta, while Farmer's observations on Pallavicinia 

 lead to the same result for one of the Bryophyta. As 

 regards the Thallophytes, it still remains as a question for 

 future observation to decide, how, and at what time there 

 follows the reduction in number of chromosomes after they 

 have been doubled by sexual coalescence, a reduction which 

 must undoubtedly occur. Strasburger thinks it probable 

 that in these low forms the reduction takes place during 

 germination, though, possibly, in such plants as Colcochcete 

 and (Edogonium, not until the development of the spores ; 

 a suggestion which seems from analogy a very probable 

 one. 



The whole tendency of observations is thus towards the 

 conclusion that the nuclei of the two alternating generations 

 differ in number of chromosomes. The cells of the sexual 

 generation showing on their division only half as many 

 chromosomes as those of the neutral generation, while the 

 critical points in the life-cycle as regards this difference 

 are — the zygote, where coalescence of the two sexual nuclei 

 results in a doubling of the chromosomes, and the spore- 



