2l6 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



weeds of cultivation are concerned, not forgetting the more devastat- 

 ing diseases of those crops. We must also consider the effects of 

 cultivation — and of the removal of Man's protection, whether such 

 protection had been intentional, for crops, or unintentional, for 

 weeds. 



Effects of Cultivation 



It is a common experience of farmers and gardeners that cultivated 

 plants and the weeds infesting them or their ground tend to lack 

 the ability to spread independently of Man and, in many cases, 

 even to maintain themselves unaided. This inability to hold their 

 own in the face of natural forces — including, in particular, competi- 

 tion from other plants — may even be observed within the natural 

 region and habitat range of the more immediate ancestors of the 

 cultivated plant or weed. The reason is that Man's influence on 

 a plant under cultivation is apt so to change its genetical make-up, 

 structure, and physiological capabilities that it is deprived of ' key ' 

 advantages in the general struggle for existence. These advantages 

 will have been acquired, often by the rigours of natural selection, 

 in preceding periods of the plant's evolutionary history, but may 

 be lost overnight, as it were, by artificial selection or, more gradually, 

 by the ' protection ' afforded by numerous generations of cultivation. 



We may here note a ' round dozen ' of the many and diverse types 

 of changes wrought by Man in the make-up and structure of cul- 

 tivated (and infesting) plants. 



(i) Genetical changes — involving the loss of characters that are 

 obviously beneficial and often needed for the plants' survival in 

 natural conditions. Besides the examples indicated among the 

 following categories, and plentiful others resulting from such 

 activities as artificial hybridization and the induction of polyploidy, 

 there are the numerous instances of physiological or otherwise less 

 structurally obvious but fundamental hereditary changes. 



(2) Physiological changes — which are commonly hereditary and 

 hence included in the above category, but which in other instances 

 manifest themselves in the life of a single generation and cause it 

 to ' pay the price '. Thus, for example, plants that have not been 

 suitably hardened may succumb on transplantation to a less favour- 

 able habitat than the one in which they originally developed and 

 to which they had become accustomed. 



(3) Structural changes — often bound up with physiological ones. 



