Chap. VIII. ON CLBISTOGAMIO FLOWEES. 343 



darkness of the changing season, whilst with others 

 it is diminished by the increasing heat and light.* 



The belief that the first step towards flowers being 

 rendered cleistogamic was due to the conditions to 

 which they were exposed, is supported by the fact 

 of various plants belonging to this class either not pro- 

 ducing their cleistogamic flowers under certain condi- 

 tions, or, on the other hand, producing them to the 

 complete exclusion of the perfect ones. Thus some 

 species of Viola do not bear cleistogamic flowers when 

 growing on the lowlands or in certain districts. Other 

 plants when cultivated have failed to produce perfect 

 flowers during several successive years ; and this is 

 the case with Juncus hufonius in its native land of 

 Eussia. Cleistogamic flowers are produced by some 

 species late and by others early in the season ; and this 

 agrees with the view that the first step towards their de- 

 velopment was due to climate ; though the periods at 

 which the two sorts of flowers now appear must since 

 have become much more distinctly defined. We do not 

 know whether too low or too high a temperature or the 

 amount of light acts in a direct manner on the size of 

 the corolla, or indirectly through the male organs being 

 first affected. However this may be, if a plant were 

 prevented either early or late in the season from fully 

 expanding its corolla, with some reduction in its size, 

 but with no loss of the power of self-fertilisation, then 

 natural selection might well complete the work and 



* For the statement by Linnajus, p. 543) "that when the autumn 



see Mohl in ' Bot. Zeituiig,' 1863, draws on, and habitually in winter 



p. b27. Asa Gray, ' American for such of our wild flowers as 



Journal of Science,' 2nd Sfries, blossom at that season," the flowers 



vol. xxxix. 18t)5, p. i05. Bennett are self-fertilised. On Lysima- 



in 'Nature,' Nov. 1869, p. 11. cliia, H. Miiller, 'Nature,' Sept, 



The Eev. (J. Henslow also says 1873, p, 433, Buuche', ' Sitzunga- 



(' Gardener's Clironicle,' 1877, p, bericht dei- Gesell. Naturforscu. 



271 : also ' Nature,' Oct, 19, 1876, Freunde ' Oct. 1874, p. 90. 



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