TRANSFERENCE OF POLLEN 281 



anthers of flowers which have opened previously, their own 

 anthers being not yet ripe when the stigma is fully developed. 



(iii) Among monoclinous flowers which are homogamous, that 

 is, which develop and ripen their sexual organs simultaneously, 

 the distance apart or.the relative position of the anthers and the 

 stigma is often such that the transference of pollen from the 

 former to the latter is rendered uncertain : examples exhibiting 

 adaptations of this class are met with in the primrose and cowslip. 



(iv) Among certain plants, especially some orchids, the pollen 

 has no fertilising effect upon ovules produced in the same flower. 



Transference of pollen. — Since the pollen-grains of plants have 

 no power of spontaneous movement, they must be carried from 

 one flower to another by some external agency. 



In certain cases snails, birds, and currents of water effect the 

 transference of pollen from place to place, but the chief agents 

 which carry the pollen-grains from one flower to another are 

 (i) the wind and (2) insects. 



Flowers which are cross-pollinated by aid of the wind are said 

 to be anemophilous or wind-poUinated : those in which the pollina- 

 tion is brought about by the agency of insects are described as 

 entomophilous or insect-pollinated flowers. 



Wind-pollinated flowers are sometimes loosely described as 

 wind-fertilised and insect-pollinated flowers as insect-fertilised; 

 it must, however, be clearly understood that the function of 

 the wind and insects is merely the transference of the pollen- 

 grains from the anthers of one flower to the stigma of another, 

 and that these agents have no direct influence upon the act of 

 fertilisation which subsequently takes place in the ovule. 



As examples of plants whose flowers are wind-pollinated may be 

 mentioned the hop, docks, almost all grasses and sedges, and many 

 trees and shrubs, such as hazel and birch. Their flowers are gener- 

 ally small and inconspicuous, without scent : ' honey ' is generally 

 absent, and the pollen-grains, which are usually produced in large 

 quantities, have a smooth and dry external surface. The anthers 



