INTRODUCTION 13 



the torrid to the polar, in every soil from rich river- 

 beds to sandy deserts, and under varying degrees 

 of nurture from oozy swamps to arid deserts. 



Plants have to fit themselves to these various loca- 

 tions, and to the differences of season, or else they 

 must die. To survive they must be able constantly 

 to adjust themselves to changing conditions. Now 

 it has been shown by thousands of experiments that 

 cross-fertilisation produces very numerous and 

 thrifty seedlings, part of which may so differ from 

 either parent that they will survive where either 

 parent might not. Cross-fertilisation so improves 

 the vigour and the adaptability of the young plants 

 that they may spread into new districts and climates, 

 surviving and increasing in numbers in spite of the 

 great struggle they must wage. 



Cross-fertilisation, then, is of vital and over- 

 whelming importance, which the plants seem to un- 

 derstand. Consequently great pains and ingenuity 

 are expended to accomplish this desideratum. 



There are various methods for procuring cross- 

 fertilisation, which may be grouped as follows : 



Staminate and pistillate flowers (dioecious 



and monoecious ) . 

 Prepotence of foreign pollen over that of 



home production. 



