MEANING OF ‘“‘ ADAPTATION.” 25 
rooted annual grass of the same community during a period, even 
short, of drought. Nor in the rain-forest can the housekeeping of 
the tall tree be the same as that of the low tree growing beneath 
its shade. So, too, In a swamp—the conditions with regard to 
light differ greatly according to the size of the plants. The New 
Zealand flax (Phormium), whose leaves need be thick to battle with 
the breeze, lives under conditions quite unknown to the creeping- 
herb, Gratiola peruviana, of the wet ground beneath. 
The term “adaptation”? has been already used in this chapter ; 
it requires some explanation. First of all, as used here, there is no 
idea of *‘ purpose ’’ underlying the expression. A root, for instance, 
is not an organ for the purpose of fastening the plant to the ground 
and taking water from the soil and conducting such water to the 
stem: it is an organ which functions in that manner. If it appears 
well suited both in form and action to carry on its special work, then 
it may be said to be an adaptation whose function it is to play 
the triple role indicated above. In some cases roots may play quite 
different parts. Thus the various lianes which climb by means of 
special roots put these forth from their elongated stems and so anchor 
the latter to the support. Here the root does not go downwards, 
nor has it any relation to water ; indeed, when its anchoring function 
b 
is no longer necessary it dies—not because it is no longer required, 
its “ purpose ”’ being fulfilled, but because the conditions under which 
such roots are given off have ceased to be, through the thick growth 
of the liane’s stem, which no longer closely hugs the tree. Such 
fastening-roots are an adaptation, but not a special one, any more 
than the usual root form is special; commonness does not degrade 
either a function or the form of an organ. There are many other 
adaptations found amongst roots, and some receive attention in 
other parts of this book. 
The relation of a plant to its water-supply is one which stands 
very plainly reflected in many plants, so that adaptations are 
readily studied in this regard. Thus, to refer to the root again, 
roots may be short (as in many annuals); they may go straight 
down for a great depth (as in the cultivated lucerne — Medicago 
sativa) ; they may extend laterally just beneath the surface of the 
soil (as in the New Zealand flax—Phormium tenax); they may con- 
sist largely of a spongy, water-absorbing tissue (as in the perching- 
orchids) ; they may possess an abundance of root-hairs (as in many 
