PLANTS AND WATER 135 



conform to its requirements, and to tolerate 

 the limitation which it imposes. 



It is necessary, however, to observe the 

 greatest caution in concluding, as is sometimes 

 done, that the various " adaptations " to 

 dry conditions are to be attributed offhand 

 to a faculty assumed to be possessed by the 

 plant which enables it to make a direct and 

 appropriate response to the demands of the 

 environment in question. As a matter of 

 fact many plants Are incapable of making 

 any purposive response at all ; and the matter 

 is by no means a simple one even in the case 

 of those which can so react. The constitution 

 of the living protoplasm is the main factor 

 which determines the nature of response to 

 water requirements, no less than to mechanical 

 needs. It is only those plants, the living 

 substance of which has become definitely 

 altered in certain (but alternative) ways, that 

 are capable of exhibiting adaptations (or 

 adaptedness) towards a particular set of 

 external conditions. The successful reaction 

 is commonly bound up with complex internal 

 functional relations, and among these nutrition 

 often plays a leading share. It may happen, 

 as in the formation of winter bud scales (see 

 p. 142), that the functional conditions which 

 are more immediately concerned in the forma- 

 tion of " adaptive " structures ensure their 

 production quite independently of their ulti- 

 mate utility as protective organs during the 

 winter months. 



