264 THE STRUCTURE OF FLOWERS. 



The other example is Buellia tuhei'osa, of which Mr. 

 Darwin remarks, " It produces both open and cleistogamous 

 flowers; the latter yield from 18 to 24, whilst the former 

 onlj from 8 to 10 seeds : these two kinds of flowers are pro- 

 duced simultaneously, whereas in several other members of 

 the family the cleistogamous ones appear only during the hot 

 season." From this one would infer that an excess of heat 

 may be a cause of cleistogamy, just as too low a temperature 

 appears to bring it about. 



I think it probable that other influences than tempera- 

 ture may be brought to bear upon a plant ; which, indeed, 

 we may see in our own Yiolets. The larger flowers of this 

 species are 7iot produced ia the hottest time of the year, 

 while the cleistogamous buds are only borne in the summer. 

 On the other hand, the foliage is only developed fully, con- 

 temporaneously with the dwarfing of the floral organs. 



Again, a poor soil has been noticed as associated with 

 cleistogamy by Torrey and Gvaj, in the case of North 

 American species of Heliantliemum. 



Temperature, however, seems to be the most important 

 agent; thus, while the climate of South Italy can develop 

 the perfect flowers and render them fertile, there cleistogamy 

 is suppressed; here, in England, the climate is seemingly not 

 sufficiently warm to do so, and the cleistogamous buds appear 

 in compensation. The vegetative energy, however, comes 

 to the fore during the summer, and perfect flowers are not 

 produced simultaneously with it ; so that it is not until the 

 vegetative period has ceased, and the materials are remade for 

 their development, that larger flowers are again borne later in 

 the year, as in November, as well as in the following spring. 



With regard to the anemophilous genera, Mr. Darwin 

 mentions Hordeumy Cryptostachys, Leersia oryzoides, and Juncus 

 hufonius in Russia. 



