1899] ASEXUAL NUCLEAR FUSLONS 239 



Asexual Nuclear Fusions. 



Fusion of nuclei, whether it accompanies the union of so-called sexual 

 cells, or occupies a position in the life-history which apparently denies 

 it that dignity, must for some time remain a subject of absorbing 

 interest, not only on account of its complexity, but also owing to the 

 important biological questions involved. 



Professor Percy Groom draws attention in a recent paper {Trans. 

 Bot. Soc. Edin. 1S9S-99, pp. 132-144) to the number of such fusions 

 of other than a distinctly sexual character, which we now know to 

 occur in the vegetable kingdom. 



Among fungi, in the Uredineae and Ustilagineae, the union takes 

 place in the teleutospore, which, originally binucleate, contains but one 

 nucleus at the period of germination, when it gives rise to the short 

 sporidium-bearing promycelium. In Proto- and Autobasidiomycetes 

 the fusion takes place in the homologue of the teleutospore, viz. the 

 young basidium, which, when mature, represents, according to Brefeld, 

 the Uredine promycelium, and bears basidiospores. Finally, in Ascomy- 

 cetes the same phenomenon may be observed in the young ascus, which 

 de Bary regards as a reduced sporophytic generation parasitic on the 

 parent plant. Apart from fungi similar nuclear fusions are only 

 known to occur among Angiosperms, where the union of two polar 

 nuclei in the embryo sac precedes the formation of the endosperm, 

 which, by the way, we are pleased to see the Professor regards as 

 homologous with that of Gymnosperms, and consequently with the 

 prothallus of the lower forms, its appearance having been postponed 

 owing to functional degeneration. These fusions are thus always 

 interpolations, and distinctly asexual in character, as is shown by 

 the position they occupy in the life-history of such forms as the 

 Ascomycete Sphaerotheca and the Angiosperms, in both of which the 

 union takes place along with and subsequent to a well-marked sexual 

 act, viz. the union of the antheridial and oogonial nuclei in the former, 

 and that of the nuclei of the pollen-tube and egg-cell in the latter. 

 In every known case they take place in a portion of the life-history, 

 which has undergone degeneration, and which is at the same time 

 fructificative in development, as well as frequently parasitic in 

 character and sometimes at least homologous with the host plant (?). 



Professor Groom suggests that if this fusion can be taken as 

 evidence of vegetative degeneration in one segment of the life cycle, 

 it may be possible to employ it as a means of distinguishing between 

 antithetic and homologous alternation of generations among plants ; but 

 whatever be the physiological rationale of such fusions — and an 

 adequate explanation seems still far to seek — they appear to have 

 much in common with the similar phenomena which constantly accom- 

 pany the union of sexual cells, and both will in all probability be 

 ultimately found to perform similar functions in the life of the plant. 



