398 THE BIOLOGY OF FLOWERING PLANTS 



§ 1 6. Pollination — General Considerations 



Looking back over this account of pollination, we see 

 that, in the vast majority of cases, the passive microspore is 

 carried to the stigma (or the micropyle in the case of the 

 gymnosperms) either by insects or by the wind. We 

 cannot say that one of these methods is better than the 

 other. The number of species which are entomophilous 

 is the greater, but probably not the number of individuals. 

 The most successful dicotyledonous family, the Com- 

 positae, is entomophilous, the most successful monocotyle- 

 donous, the Gramineae, is anemophilous. Yet the influence 

 which has directed the course of evolution towards the 

 production of the gay, fragrant blossom, which we naturally 

 associate with the phrase " flowering plant," has been the 

 relation of the insect to pollination. 



Not only is pollen transferred, it tends to be transferred 

 in away which, at the least, makes cross-polhnation possible. 

 In anemophilous plants the means are chiefly dicliny and 

 dichogamy ; these are employed, too, in entomophilous 

 plants, but much more striking in these are the floral 

 mechanisms. We must admire the variety and apparent 

 ingenuity of these, but we are not yet in a position to 

 evaluate their net importance, efficiency, and necessity. 

 They are very widespread. In some cases they restrict 

 polHnation to one particular method, as in the red clover, 

 which sets seed only when visited by the humble bee ; 

 when this crop was introduced into New Zealand no seed 

 was set till the humble bee, too, was introduced. On the 

 other hand, Kirchner (1922) has shown that of the hundred 

 odd European orchids, fifteen are habitually self-pollinated. 

 The same is known to be true of about 150 exotic species. 

 These are flowers of the type most highly specialised in 

 relation to insect visits, and the autogamous species appear, 

 in most cases, to be as much suited structurally as the 

 others to insect polHnation, and have indeed been fre- 

 quently described as insect-pollinated. If this is so in a 

 highly specialised flower, it is clear that only careful 



