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. 
