62 VIOLA. [CHAP. 



portions of the two middle stamens ? — Because this 

 enables the bee to move the pistil, and thereby to 

 set free the pollen more easily than would be the 

 case under the reverse arrangement. 



In Viola tricolor, the form of the stigma is very 

 different from that of V. canina, but the reason of the 

 difference has not been satisfactorily explained. Mr. 

 Bennett considers that tftis species is fertilised by 

 Thrips. Mr. Darwin, however, has satisfied himself 

 that when bees are excluded, it is comparatively 

 infertile, and he has favoured me with the following 

 memorandum on the subject. 



" When," he says, " I formerly covered up a fine, 

 large, cultivated variety, it set only i8 capsules, and 

 most of them contained very few good seeds, several 

 from only i to 3 ; whereas an equally fine- uncovered 

 plant, growing close by, produced 105 fine capsules. 

 The few capsules which are produced when insects 

 are excluded are probably due to the curling up of 

 the petals (as Fermond and F. Mijller remark) as they 

 wither, by which process pollen-grains adhering to the 

 papillae may be inserted into the cavity of the stigma. 

 The moth Plusia is said to visit the flowers largely. 

 Humble-bees are common agents in fertilising these 

 flowers ; but I have seen more than once a fly 

 {Rhyngia rostrata) with the under side of its body, 

 head, and legs dusted with the pollen of this plant, 

 and having marked the flowers which they had visited, 

 found, after a few days, that they had all been fertilised. 



" It is curious in this case, as in many others, how 

 long the flowers may be watched without seeing one 

 visited by an insect. During one summer, I repeatedly 



