CHAP IV VIOLA TRICOLOR. 125 



fertilisation, and as they are not visited by insects nearly so often 

 as most other nectar-secreting flowers, we can understand the 

 remarkable fact discovered by H. Miiller and described by him 

 in ' Nature,' namely, that this species exists under two forms. 

 One of these bears conspicuous flowers, which, as we have seen, 

 require the aid of insects, and are adapted to be cross-fer- 

 tilised by them ; whilst the other form has much smaller and 

 less conspicuously coloured flowers, which are constructed on a 

 slightly different plan, favouring self-fertilisation, and are thus 

 adapted to ensure the propagation of the species. The self- 

 fertile form, however, is occasionally visited, and may be crossed 

 by insects, though this is rather doubtful. 



In my first experiments on Viola tricolor I was unsuccessful in 

 raising seedlings, and obtained only one full-grown crossed and 

 self-fertilised plant. The former was 12i inches and the latter 8 

 inches in height. On the following year several flowers on a 

 fresh plant were crossed with pollen from another plant, which 

 was known to be a distinct seedling; and to this point it is im- 

 portant to attend. Several other flowers on the same plant 

 were fertilised with their own pollen. The average number 

 of seeds in the ten crossed capsules was 18 '7, and in the twelve 

 self-fertilised capsules 12 '83; or as 100 to 69. These seeds, 

 after germinating on bare sand, were planted in pairs on the 

 opposite sides of five pots. They were first measured when 

 about a third of their full size, and the crossed plants then 

 averaged 3 '87 inches, and the self-fertilised only 2 '00 inches in 

 height ; or as 100 to 52. They were kept in the greenhouse, and 

 did not grow vigorously. Whilst in flower they were again 

 measured to the summits of their stems (see Table XLL), with 

 the following result : 



The average height of the fourteen crossed plants is hero 5 '58 

 inches, and that of the fourteen self-fertilised 2 '37; or as 100 to 

 42. In four out of the five pots, a crossed plant flowered before 

 any one of the self-fertilised ; as likewise occurred with the pair 

 raised during the previous year. These plants without being 

 disturbed were now turned out of their pots and planted in the 

 open ground, so as to form five separate clumps. Early in the 

 following summer (1869) they flowered profusely, and being 

 visited by humble-bees set many capsules, which were carefully 

 collected from all the plants on both sides. The crossed plants 

 produced 167 capsules, and the self- fertilised only 17; or as 

 100 to 10. So that the crossed plants were more thaix twice ^be 



