4 INTRODUCTION. 



will be described in the last chapter of this volume. 

 They are inarifestly adapted for self-fertilisation, which 

 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 ; but some few 

 other plants behave in the same manner ; and, as they 

 do not bury all their flowers, they might have formed 

 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, Rliinantlms 

 crista-galliy 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 con- 

 cerned, namely, the assured propagation of the species, 

 they approach in nature cleistogamic flowers ; but they 

 differ from them by the two kinds being produced 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- 



* H. Miiller ' Nature,' Sept. 25, ' Die Bcfruclitung dcr Blumeu, 

 1873 (vol. viii.'), p. 433, and Nov. &c., 1873, p. 20i. 

 20, 1873 (vol. ix.), p. 44. Also 



