250 GRADATION OF ORGANS. CuAP. IX. 



formed utriculi. These are believed to be connected 

 with the penetration of the pollen-tubes; and their 

 absence in the rostellum probably accounts for its 

 not being penetrated. If the structure of the ros- 

 tellum and of the stigma is as here described, their 

 only difference consists in the layer of cells which 

 secrete the viscid matter being thicker in the ros- 

 tellum than in the stigma, and in the utriculi having 

 disappeared from the former. There is therefore no 

 great difficulty in believing that the upper stigma, 

 whilst still in some degree fertile or capable of pene- 

 tration by the pollen-tubes, might have gradually 

 acquired the power of secreting a larger amount of 

 viscid matter, losing at the same time its capacity for 

 fertilisation ; and that insects smeared with this viscid 

 matter removed and transported the pollen-masses in 

 a more and more effective manner to the stigmas of 

 other flowers. In this ease an incipient rostellum 

 would have been formed. 



In the several tribes, the rostellum presents a 

 marvellous amount of diversity of structure ; but most 

 of the differences can be connected without very wide 

 breaks. One of the most striking differences is, that 

 either the whole anterior surface to some depth, or 

 only the internal parts become viscid ; and in this 

 latter case the surface retains, as in Orchis, a mem- 

 branous condition. But these two states graduate 

 into each other so closely, that it is scarcely possible 

 to draw any line of separation between them : thus, 

 in Epipactis, the exterior surface undergoes a vast 

 change from its early cellular condition, for it becomes 

 converted into a highly elastic and tender membrane, 

 which is in itself slightly viscid, and allows the under- 

 lying viscid matter readily to exude ; yet it acts as a 

 membrane, and its under surface is lined with much 



