VI] FIXITY OF POSITION IN PLANTS 91 



risks ill various ways : such as by migration, or by 

 retiring into nests or burrows. The fixed plant has 

 to remain where it is. It may hibernate, however, 

 underground, as do many herbaceous and bulbous 

 plants ; or it may reduce its physiological risks by the 

 fall of the leaf, as do our ordinary deciduous trees in 

 the autumn ; or it may elude the period of extreme 

 conditions in the form of seeds or resting spores, 

 which are shown to be more resistant than the 

 actively vegetating plant. But it would take too 

 long to do more than merely suggest these as some 

 of the many devices by which plants overcome the 

 difficulties of climatic stress, to which their fixed 

 position condemns them. 



A still more weighty disability than those which we 

 have thus seen to follow on the adoption of a fixed 

 position, is the difficulty in effecting pollination, and 

 its usual concomitant intercrossing. This is not the 

 place to discuss the advantage which follows on cross- 

 fertilisation in plants. It may be accepted as a general 

 principle that advantage does accrue. A general 

 reference may be given to Darwin's book on The 

 Effects of Cross and Self-fertilisation in the Vegetable 

 Kingdom. He there states, in his last chapter, that 

 "the first and most important of the conclusions 

 which may be drawn from the observations given 

 in this volume is that cross-fertilisation is generally 

 beneficial, and self-fertilisation injurious." In am- 



