CHAP. VIII. ON CLEISTOGAMIC FLOWERS. 341 



flowers of Viola are indeed sterile unless they are 

 visited by bees ; but when thus visited they yield the 

 full number of seeds. As far as I have been able to 

 discover thore is only one absolute exception to the 

 rule that the perfect flowers are fertile, namely, that 

 of Voandzeia ; and in this case we should remember 

 that cultivation often affects injuriously the repro- 

 ductive organs. Although the perfect flowers of 

 Leersia sometimes yield seeds, yet this occurs so 

 rarely, as far as hitherto observed, that it practically 

 forms a second exception to the rule. 



As cleistogamic flowers are invariably fertilised, and 

 as they are produced in large numbers, they yield 

 altogether a much larger supply of seeds than do 

 the perfect flowers on the same plant. But the latter 

 flowers will occasionally -be cross-fertilised, and their 

 offspring will thus be invigorated, as we may infer 

 from a wide-spread analogy. But of such invigoration 

 I have only a small amount of direct evidence: two 

 crossed seedlings of Ononis minutis&ima were put into 

 competition with two seedlings raised from cleisto- 

 gamic flowers ; they were at first all of equal height ; 

 the crossed were then slightly beaten ; but on the fol- 

 lowing year they showed the usual superiority of their 

 class, and were to the self-fertilised plants of cleisto- 

 gamic origin as 100 to 88 in mean height. With 

 Vandellia twenty crossed plants exceeded in height 

 twenty plants raised from cleistogamic seeds only by 

 a little, namely, in the ratio of 100 to 94. 



It is a natural inquiry how so many plants belong* 

 ing to various very distinct families first came to have 

 the development of their flowers arrested, so as ulti- 

 mately to become cleistogamic. That a passage from 

 the one state to the other is far from difficult is shown 

 by the many recorded cases of gradations between the 



