226 Plant Life and Evolution 



nearly all gymnosperms and the apetalous angio- 

 sperms like the oaks and most grasses, are gen- 

 erally dependent upon the wind for distributing their 

 pollen, which is light and produced in very great 

 quantities, and is readily borne long distances 

 through the air. Cross-pollination is often the rule, 

 however, even here, as the flowers are frequently 

 " diclinous," that is, have their stamens and carpels 

 in different flowers. Wind pollination involves a 

 great waste of pollen, as probably not one pollen- 

 spore in a million is efficacious. It is clear that a 

 material saving in the amount of pollen and its in- 

 creased efficiency ought to be of advantage to the 

 plant. 



Pollen cells are rich in nutritive matter, and hence 

 are sought for as food by many insects. It may 

 be supposed that the first cases of insect pollination 

 were purely accidental and brought about by the 

 search for pollen as food. If for any reason any 

 flowers should be more conspicuous than others, it 

 is quite conceivable that they would more readily 

 attract the attention of visiting insects, and it is 

 quite conceivable also that through some increased 

 size of the enveloping leaves, or brighter color of 

 the stamens, the line of evolution started which 

 culminated in the gorgeously colored and highly 

 specialized flowers of many of the orchids and Com- 

 positse. 



In the lower types of flowers the enveloping 

 leaves are inconspicuous scales, serving merely 



