THE FLOWER 241 



The existence of this law once admitted, the signifi- 

 cance of a number of peculiarities in the structure of 

 the flower, otherwise inexplicable, becomes clear. A 

 great number of facts have been accumulated in 

 botanical literature in support of the theory that 

 the structure of flowers is adapted to cross-fertilisa- 

 tion, especially through the agency of insects. Let us 

 here dwell only on the most prominent outlines of this 

 theory. 



The following considerations can be brought forward in 

 support of the theory that a showy appearance, perfume, 

 and honey glands exist for the purpose of attracting 

 insects, which in flying from one flower to the other 

 promote cross-fertilisation. In the first place, all these 

 characters exist only during the period of pollination, 

 after which they disappear. Secondly, there is a marked 

 difference between plants in which pollination is effected 

 by means of wind and those in which it is effected 

 through the agency of insects. Flowers of the former 

 kind are generally small and unsightly, are never highly 

 coloured, and possess neither perfume nor honey. Such 

 are the majority of trees, e.g. the pine, the birch, the 

 aspen, and so forth. On the other hand, these plants 

 produce an abundance of pollen. Clouds of yellow 

 dust falling on the soil or on the surface of water have 

 even given rise to superstitious beliefs in rains of sul- 

 phur. It is noteworthy that such flowers generally 

 bloom in early spring, when the undeveloped leaves 

 cannot prevent the scattering of the pollen. 



It is clear, nevertheless, that such a waste of material 

 probably precious to the plant, a waste inevitable during 

 pollination by the wind, must be a very great disad- 

 vantage to the plant. Apparently it is more profitable 

 to a plant to produce less pollen and to secure more 

 accurately its transference to the stigmas of other plants. 

 This economical purpose is evidently served by insects. 

 Honey glands, the so-called nectaries, entice insects to 



Q 



