Chapter XVIII 



PLANT ADJUSTMENTS AND APPLICATIONS 



Consideration of both (i) the natural ' adaptations ' and (2) the 

 man-made modifications of plants, in each case (a) of individual 

 kinds and (b) of vegetation, gives us by cross-inference four sets of 

 topics. These are all of peculiar interest and importance, aud are 

 exemplified individually by (/' ) evolution (which depends substantially 

 on the favourable modifications or adaptations of individual plants) ; 

 [ii) plant-breeding (in which Man manipulates the characteristics 

 of individual plants) ; {Hi) successional change (of natural vegetation) ; 

 and {iv) combating of erosion (for which planted or encouraged 

 vegetation is the best weapon). To the extent that these and allied 

 considerations are often areal, for example in leading to extended 

 ranges, they should be included among the concerns of plant geo- 

 graphy. Each is a large subject that can be treated here only in 

 broad outline or with special reference to the example mentioned. 



' Adaptations ' of Individuals 



However evolution of plants may have come about — and there 

 appear to be many and various causes and mechanisms of evolu- 

 tionary change — it is clear that the tendency is generally towards 

 forms more suitably adapted to the environment than were their 

 ancestors. Actually it seems that the main single cause of evolution 

 in plants has been — and continues to be — the ' natural selection ' 

 of heritable characteristics that are beneficial in competition and 

 hence advantageous in the general ' struggle for existence '. Most 

 species produce, at least potentially in the form of disseminules, 

 far more individuals than ever survive, and it is the progeny pos- 

 sessing favourable variations in structure or function that tend to 

 persist. When these variations are heritable, they mav become 

 ' fixed ' in succeeding generations and benefit the race ; for in the 

 intense competition that is characteristic of life on earth, individuals 

 having even slight advantages over their fellows, for example in 

 exhibiting taller or more rapid growth, will have the best chances 



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