318 THE STRUCTURE OF FLOWERS. 



the action of insects. I have given reasons for believing, and 

 the reader can readily suggest other instances, that structural 

 peculiarities have grown in response to pressures and thrusts 

 made upon the floral organs by the insects themselves ; 

 and that such have sometimes produced protuberances or 

 obstructions in the way of the emission of the pollen upon 

 the stigma of the same flower, is no more than might be 

 anticipated to be extremely probable. Thus one of the 

 most remarkable is the rostellum of Orchids, believed to be 

 a modified stigma now converted to a new use. In nearly 

 all Orchids this blocks up the way of access to the stigmatic 

 chamber, while the pollen masses recline on the roof over 

 it, so to say ; but when Orchids become self-fertilising or 

 even cleistogamous as well, this is often brought about by the 

 degradation of the rostellum ; so that the pollen masses can 

 then easily slide over the summit of the stigmatic chamber 

 and fall into it at once. When they do so they are fully 

 self-fertile, as Mr. Henry O. Forbes has shown.* 



Some few plants are quite barren with their own pollen, 

 even when artificially placed upon the stigma ; though 

 Lobelia and Digitalis do not belong to the group. These, 

 as shown elsewhere, can and often do become fully fertile 

 at other places and seasons, and are thereby benefited by 

 acquiring the possibility of setting seed by self-fertilisation, 

 as otherwise they might set none at all. 



There are, then, three kinds of barriers to self-fertilisation : 

 one mechanical, as in Orchids ; a second, that of time, when 

 e.g. a flower is so strongly protandrous that the pollen is all 

 shed before the stigmas are mature ; and, thirdly, a physiolo- 

 gical one, when the pollen is actually impotent on the 

 stigma of the same flower, even though it be homogamous. 



* On the Contrivances for insuring Self-fertilisation in some Tropical 

 Orchids, Journ. Linn. Soc, xxi., p. 538. 



