4 INTRODUCTION. 



is effected at the cost of a wonderfully small expendi- 

 ture of pollen; whilst the perfect flowers produced by 

 the same plant are capable of cross-fertilisation. Cer- 

 tain aquatic species, when they flower beneath the 

 water, keep their corollas closed, apparently to protect 

 their pollen; they might therefore be called cleisto- 

 gamic, but for reasons assigned in the proper place are 

 not included in the present sub-group. Several cleis- 

 togamic species, as we shall hereafter see, bury their 

 ovaries or young capsules in the ground. Some few 

 plants produce subterranean flowers, as well as ordinary 

 ones; and these might have been formed into a small 

 separate subdivision. 



Another interesting subdivision consists of certain 

 plants, discovered by H. Miiller, some individuals 

 of which bear conspicuous flowers adapted for cross- 

 fertilisation by the aid of insects, and others much 

 smaller and less conspicuous flowers, which have often 

 been slightly modified so as to ensure self-fertilisation. 

 Lysimachia vulgaris, Euphrasia officinalis, Rhinan- 

 thus crista-galli, and Viola tricolor come under this 

 head.* The smaller and less conspicuous flowers are 

 not closed, but as far as the purpose which they serve 

 is concerned, namely, the assured propagation of the 

 species, they approach in nature cleistogamic flowers; 

 but they differ from them by the two kinds being pro- 

 duced on distinct plants. 



With many plants, the flowers towards the outside of 

 the inflorescence are much larger and more conspicu- 

 ous than the central ones. As I shall not have occa- 

 sion to refer to plants of this kind in the following 

 chapters, I will here give a few details respecting them. 



der 



