1893.] the origin of this and similar phenomena. 133 



Dicecism, in angiosperms, is descended from hermaphroditism, 

 and although it is hereditary, it has been shown that the sex 

 of a seedling can to some extent be determined in advance by 

 its conditions of nutrition. Thickly sown^ seedlings of Lychnis 

 diurna gave 200 males to 100 females. Thinly sown, on the 

 other hand, 77 males were obtained per 100 females. Various 

 similar observations have been made. The phenomena of dioecism 

 show considerable variation in nature, like those of gynodioecism 

 (see Darwin, loc. cit). 



Fragaria, hermaphrodite in England, becomes dioecious or 

 polygamous in the United States. Dioecism in some cases may 

 have come from heterostylism (Mitchella, &c.). Monoecism presents 

 similar phenomena. 



Cleistogamy is also a very variable phenomenon, appearing 

 sporadically in many plants, constantly in others. It appears to 

 vary with the time of year, soil, climate, light and temperature 

 (according to some unpublished observations of the author's upon 

 Salvia Verbenaca L.). Vochting'^ has recently shown in a very 

 striking way the effect of light upon the production of cleisto- 

 gamy. It is noteworthy that cleistogamous plants are not usually 

 very dichogamous. It does not usually occur in gynodioecious 

 genera (except Salvia). 



Lastly dichogamy is a phenomenon closely bound up with 

 some of the above, and largely dependent on external conditions, 



Meehan^ has tried to show that it is largely dependent on 

 temperature, and the author inclines to the same view, though 

 in a modified form. It varies with season, soil and other con- 

 ditions, both in different plants of the same species, and (as 

 observed by the author) in the same plant at different times. 



To sum up, it appears probable that all these phenomena are 

 closely allied to one another, depending very largely upon ex- 

 ternal causes, and capable of being called out, or modified, by 

 these, in a very marked degree, but at the same time, fixed to 

 a great extent by natural selection and heredity. The author 

 is now engaged in working up the literature relating to these 

 subjects, and hopes to publish a review of it, discussing the 

 subject of the origin of these phenomena in some detail. 



In conclusion, the author desires to thank Mr F. Darwin, 

 Mr R. I. Lynch, and Mr I. H. Burkill for valuable advice and 

 assistance rendered. 



1 Hoffmann, "Ueber Sexualitat." Bot. Zeit. vol. 43, 1885, p. 145. 



2 Prings. Jahrb. xxv., 1893, Heft 2. 



3 Various papers, chiefly in Proc. Acad. Nat. Sci. Philadelphia, 1885 — 1893. 



