Dec. 1898.] BOTANICAL SOCIETY OF EDINBUllGH 133 



yet offered can be deemed satisfactory Le Monnier 

 and, after him, Conway Mac^Iillan (2) hold the view tliat 

 tlie nuclear fusion in question is a true sexual act, 

 and that the endosperm represents a small sporophyte 

 which is sister to the main sporophyte. This hypotliesis 

 implies that the ovule contains at least one macrospore 

 and one microspore, with their attendant prothallia, and 

 consequently that the carpel is a macro-micro-sporophyll ; 

 or it implies that the prothallium is hermaphrodite. 

 Further, unless a number of additional assumptions be 

 made, it denies the homology of the endosperm of the 

 various Angiosperms and the Gymnosperms, either with 

 one another, or else with the female prothallium of the 

 Vascular Cryptogams. Another hypothesis is put forward 

 by G. Mann, who regards the eight nuclei as representing 

 eight female cells, two of which coalesce to form the 

 definitive nucleus of the embryo-sac. 



In endeavouring to solve the problem as to the signi- 

 ficance of this peculiar fusion of the polar nuclei in the 

 embryo-sac, it is natural to ask, " Do nuclear unions of 

 equally obscure nature occur in plants other than Angio- 

 sperms ? And, if so, is it possible to draw any general 

 conclusions from a comparison among all the known cases 

 of nuclear unions ? " In answer to the first question, it 

 may be pointed out that, thanks to the labours of 

 Dangeard, H. Wager, Sapiu-Trouffy, Eaciborski, and 

 Harper, definite nuclear unions are known to occur in 

 Ascomycetes, Basidiomycetes, Uredinea?, and Ustilaginese, 

 between pairs of nuclei inside cells which show no sexual 

 differentiation. The sole explanation hitherto offered in 

 reference to these nuclear unions, occurring in the fungi 

 named, has been that they are sexual acts. The first blow 

 to this hypothesis, so far as it concerns the Ascomycetes, 

 was dealt by Harper (1). His researches on Sphaerotheca 

 Caslarjnci led him to the conclusion that the fusion of the 

 two nuclei in the young ascus is preceded by a nuclear 

 union denoting the true sexual act. In particular, Harper 

 states that the single nucleus of the ascogonium coalesces 

 with a nucleus derived from a contiguous antheridium, 

 whereupon the ascogonium divides transversely to produce 

 a row of cells, and one binucleate coenocyte which becomes 



