CHAP. X. PREPOTENT POLLEN. 393 



In some few cases the special movements of certain 

 organs almost ensure pollen being carried from plant 

 to plant. Thus with many orchids, the pollen-masses 

 after becoming attached to the head or proboscis of an 

 insect do not move into the proper position for striking 

 the stigma, until ample time has elapsed for the insect 

 to fly to another plant. With Spiranthes autumnalis, 

 the pollen-masses cannot be applied to the stigma until 

 the labellum and rostellum have moved apart, and 

 this movement is very slow.* With Posoqueria fra- 

 grans (one of the Kubiaceae) the same end is gained by 

 the movement of a specially constructed stamen, as 

 described by Fritz Miiller. 



We now come to a far more general and therefore 

 more important means by which the mutual fertilisation 

 of distinct plants is effected, namely, the fertilising 

 power of pollen from another variety or individual 

 being greater than that of a plant's own pollen. The 

 simplest and best known case of prepotent action 

 in pollen, though it does not bear directly on our 

 present subject, is that of a plant's own pollen over that 

 from a distinct species. If pollen from a distinct species 

 be placed on the stigma of a castrated flower, and then 

 after the interval of several hours, pollen from the same 

 species be placed on the stigma, the effects of the 

 former are wholly obliterated, excepting in some rare 

 cases. If two varieties are treated in the same manner, 

 the result is analogous, though of a directly opposite 

 nature ; for pollen from any other variety is often or 

 generally prepotent over that from the same flower. I 

 will give some instances : the pollen of Mimulus luteus 

 regularly falls on the stigma of its own flower, for the 



'The Various Contrivances Orchids are fertilised,' 1st edit, 

 which British and Foreign p. 128. Second edit. 1877, p. 110 



