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SCIENCE 



[N. S. Vol. XXV. No. 63(5 



sporophyte— rather than as giving any 

 evidence of sexual character. The experi- 

 ments are interesting, however, in showing 

 the extreme plasticity of the plant cell. 



There seem to be a number of different 

 stages in a sexual act which may be passed 

 through more or less independently of one 

 another. Cell union in many forms is not 

 at once followed by nuclear fusion. In the 

 Desmids, for example, the nuclei do not 

 fuse till the germination of the zygospores 

 and in the rusts a whole life cycle is in- 

 terpolated between the sexual union of cells 

 and the fusion of nuclei. The fact that in 

 certain hybrids the maternal and paternal 

 chromosomes seem to retain their individ- 

 uality throughout the whole plant up to the 

 reduction division in the formation of the 

 sexual cells, would indicate that delayed 

 fusion of chromatic substance may be a 

 common phenomenon, although generally 

 less easily detected than in the rusts. 



Not only may cell and nuclear unions 

 occur independently of each other, but a 

 distinct sexual reaction may take place 

 leading to the formation and approximation 

 of the sexual elements, though not to their 

 union. In the mueors, when a sexual race 

 of one species is grown between the male 

 and female races of a different species, the 

 sexual reaction between the races of op- 

 posite sex is shown by a white line due to 

 the accumulation of imperfect hybrids. 

 The reaction is sufficient only for the for- 

 mation and mutual attraction of the con- 

 jugative branches. The second stage— the 

 fusion of the sexual cells— occurs only 

 when the opposite sexes belong to the same 

 species and apparently can not take place 

 between the opposite sexes of different 

 species. 



In treating the evolution of sex, it is 

 customary to confine the attention to the 

 progressive differentiation of the gametes 

 alone. Differentiation of sex on separate 



individuals, whether or not accompanied 

 by a differentiation in size of the gametes, 

 would seem, however, to be the highest stage 

 reached in the development of sex. The 

 mueors may conveniently be taken as a 

 basis of our discussion of this differentia- 

 tion. There are comparatively few species 

 in which the two sexes are imited upon a 

 single plant. If we assume this her- 

 maphroditic group to be the more primi- 

 tive, we have a progressive differentiation 

 in two directions : first, to heterogamy, i. e., 

 to a constant dissimilarity in the gametes; 

 secondly, to a constant dissimilarity in the 

 sexual plants themselves. The large ma- 

 jority of the mueors have the sexes on 

 separate individuals. Since the plants are 

 capable of multiplication by non-sexual 

 spores, a single sex may be indefinitely 

 propagated non-sexually and the offspring 

 thus obtained may be spoken of as male 

 and female races. 



It is difficult to conceive of a blending of 

 characters in hermaphroditic species when 

 the gametes may come from the same 

 branch, even if they are in some species of 

 different size. In dioecious species zygo- 

 spores have been obtained from matings of 

 male and female races which in one species 

 came from as diverse regions as China and 

 Cambridge, Mass. In such forms the en- 

 vironmental conditions under which the 

 sexually opposite races have grown may be 

 sufficiently different to furnish a basis for 

 the advantages assumed to accrue from a 

 blending of maternal and paternal char- 

 acters in the offspring. 



The mueors form the only group of the 

 lower cryptogams in which the presence of 

 sexual races has been demonstrated. 



In plants a differentiation is more or less 

 apparent into a stage bearing the gametes 

 and a stage bearing the spores. The accom- 

 panying figures represent diagrammatic- 

 ally the sexual character of these two stages 

 in certain groups of plants. The stage 



