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SCIENCE 



[N. S. Vol. XXV. No. 636 



trasted with asexual spore formation, and 

 second, as to the cause or causes of the dif- 

 ferences which exist between the egg and 

 antherozoid cells which fuse and of the 

 secondary sexual characters of the individ- 

 uals which produce these cells. This dis- 

 tinction has not always been clear in the 

 work of those who have studied the so- 

 called cyclic alternation of parthenogenesis 

 and sexual reproduction in animals. 



The interesting results obtained by Klebs 

 and others on a wide variety of fungi and 

 algse show conclusively that a very high 

 degree of plasticity characterizes these 

 plants as to the time of occurrence and 

 relative predominance of asexual spore for- 

 mation and sexual reproduction by cell 

 copulations. Environmental stimuli and 

 conditions of all sorts may determine 

 whether the plant reproduces itself sexu- 

 ally or asexually or merely continues vege- 

 tative growth. We have in these simpler 

 plants numerous cases like Penicillium in 

 which sexual reproduction occurs only at 

 very long intervals or not at all, while in 

 many of the mildews, for example, a period 

 of asexual spore formation is quite regu- 

 larly followed by sexual reproduction. 

 The same differences are abundantly illus- 

 trated in the algae. It is quite possible, 

 however, as I have elsewhere pointed out, 

 that the very apparent influence of en- 

 vironmental conditions in determining 

 whether sexual or asexual reproduction 

 shall occur is, after all, only an accessory 

 manifestation and that there is really a 

 more fundamental series of cyclic changes 

 in the organism which are primarily re- 

 sponsible for the result. 



Functionally there can be little question 

 that this cyclic succession of asexual spore 

 formation followed later and under special 

 conditions by sexual reproduction is equiv- 

 alent to the alternation of parthenogenetie 

 and sexual stages in the Aphides, etc. 

 Asexual spores have the same advantages 



for the rapid spread and multiplication of 

 the plant as parthenogenetie eggs have for 

 the animal. In both cases environmental 

 conditions seem to play a large part in de- 

 termining which stage shall occur at all 

 or predominate, and in these environmental 

 conditions there are probably two sets of 

 factors to be distinguished. First, the fac- 

 tors which lead to sexual as contrasted with 

 asexual reproduction, and second, the fac- 

 tors which lead to the differentiation of the 

 male and female organisms and sex cells. 



It is quite clear in the case of the plants 

 that reproduction by conjugation is by no 

 means necessarily the fusion of visibly dif- 

 ferentiated cells. It consists in its essen- 

 tial nature and in its most primitive occur- 

 rence merely in the union of two cells, 

 whether like or unlike, in size, etc. The 

 facts of heredity give, in general, no evi- 

 dence that the gametes differ in any respect 

 in their capacity to influence the offspring, 

 and cytological data are in harmony with 

 the assumption of the essential equivalence 

 of the gametes even when their visible dif- 

 ferentiation is most conspicuous. 



In the simpler plants this functional 

 equivalence is paralleled by an entire re- 

 semblance between the gametes in size, 

 form, relative activity, etc. "We may as- 

 sume that there are inner and sexual dif- 

 ferences between the gametes of Sporo- 

 dinia, Mesocarpus, etc., but the assumption 

 is based on no observable facts of structure 

 and behavior and tends rather to mysticism 

 than to an explanation of the phenomena 

 of cell fusion. The changed relations of 

 surface area and volume, and the known 

 effects of an increase of the chromosome 

 number on cells and nuclei may just as well 

 be regarded as the first direct effects of 

 conjugation, as the satisfaction of any mys- 

 terious sexual affinity due to some invisible 

 sexual differentiation in the gametes. 



The phenomenon of prepotency is also 

 to be sharply distinguished from sexual 



