History of the Sexual Theory. [BOOK in. 



observed the same thing in Nigella arvensis, he afterwards 

 found exactly the opposite arrangement in a species of Eu- 

 phorbia, in which the stigmas can receive the pollen by the 

 aid of insects only from older flowers. 



He goes on to say that he grounds his theory of flowers on 

 these his six chief discoveries made in the course of five years ; 

 he then gives his theory at length, first of all explaining the 

 nature of juice-secreting glands (nectaries), and organs for 

 receiving or covering the nectar, and the contrivances for 

 enabling insects to find their way readily to the juice. He 

 calls attention to Koelreuter's excellent observations on the 

 fertilisation of nectar-bearing flowers by insects, and notices 

 that no one has hitherto shown that the whole structure of 

 such flowers has this object in view, and can be fully explained 

 by it. He finds the chief proof of this important proposition 

 in dichogamy. 



* After the flower,' he says, 'has opened in dichogamous 

 plants, the filaments have or assume either all at once or one 

 after another a definite position, in which the anthers open 

 and offer their pollen for fertilisation. But at this time the 

 stigma is at some distance from the anthers and is still small 

 and closed. Hence the pollen cannot be conveyed to the 

 stigma either by mechanical means or by insects, for there is 

 as yet properly no stigma. This condition of things continues 

 a certain time. When that time is elapsed, the anthers have 

 no longer any pollen, and changes take place in the filaments 

 the result of which is that the anthers no longer occupy their 

 former position. Meanwhile the pistil has so changed that 

 the stigma is now exactly in the place where the anthers were 

 before, and as it now opens, or expands the parts of which it 

 is composed, it often fills about the same space as the anthers 

 filled before. Now the spot, which was at first occupied by 

 the ripe anthers and is now occupied by the ripe stigma, is so 

 chosen in each flower, that the insect for which the flower is 

 intended cannot reach the juice without touching with a por- 



