156 POLLEN DISTRIBUTED [CH. XII 



plantain has no scent, and does not secrete nectar; in 

 fact it has none of the qualities which were referred to 

 above as serving to attract insects 1 . 



Another point is the production of great quantities of 

 pollen ; this is not so striking in the plantain as in some 

 other members of the wind-fertilised class, for instance in 

 the yew or pine, in which the clouds of dusty pollen, which 

 may be shaken out of a branch, are familiar to every 

 one. In this way pollen comes to be widely distributed, 

 and has been found in the dust collected at considerable 

 heights in the air. 



The biological meaning of this profusion of pollen is 

 clear enough: the plant has to trust to chance for the 

 conveyance of pollen from stamen to stigma, instead of 

 to the visits of insects by which small loads of pollen are 

 transferred directly from flower to flower. Thus to make 

 sure of all the countless stigmas on an oak tree being 

 dusted with pollen, enormous and apparently wasteful 

 quantities of the material must be manufactured. The 

 pollen-grains of wind-fertilised plants are smooth, dry and 

 incoherent, and seem especially adapted for floating like 

 dust in the air. 



On the other hand the pollen of insect-fertilised plants 

 is coherent like a damp powder. This quality is generally 

 due to the coats of the pollen-grains being sculptured into 

 minute prickles so that the grains cohere in groups and 

 masses; the dog-daisy supplies an instance of rough- 

 coated pollen-grains. 



1 Some species of Plantago are visited by pollen-collecting insects, 

 and are both scented and conspicuous in colour. 



