164 
XEROPHYTES 
plant-organisation is excellent, and favours rapid growth. 
But if the roots are not able to absorb water at all times 
fast enough for so much evaporation, it is evident that a 
reduction of the transpiration is a prime necessity of life, 
and that plants in which this is effected will be most 
suitable to such a position or climate. These conditions 
occur in many places — in countries with long dry seasons 
when the water supply runs short, in arctic and high moun- 
tain regions where everything favours transpiration (see 
below) and the coldness of the soil checks absorption, in 
sandy or rocky soils, upon sea-shores or in salt steppes where 
the presence of salt in the soil renders absorption diffi- 
cult, in epiphytic situations (see below) and also in winter 
when the soil is too cold for absorption. In all such places 
the plants show a remarkable general similarity, and agree 
in having a lower rate of transpiration than plants living 
where there is plenty of available water. Plants of this kind 
are termed xerophytes . 
As so often happens, a compromise has to be made 
between opposing necessities — the need of reducing evapo- 
ration, and the need of vigorous assimilation. The structural 
features that favour transpiration favour assimilation also, 
and the reduction of the former tends to be accompanied 
by that of the latter function. Every possible compromise 
may be seen in the variety of structure found in xero- 
phytes. 
All round the world, in the sub-tropical regions, there 
are vast areas in which there is a long rainless season in the 
year, e.g. the Mediterranean region, Arabia, the Steppes of 
Asiatic Russia, the desert of Gobi, the southern prairies of 
North America, Mexico, Peru, Chili, the Pampas, much of 
South Africa and Australia. In all these regions xerophytes 
occur, their degree of adaptation depending on the length 
of the dry season. If the drought be not very prolonged, 
many are able to survive by the mere reduction of the 
transpiration, but if it be extreme, or last for a long period, 
water is stored up to enable the plants to survive, and they 
are more or less fleshy in structure. In this case other 
materials are often stored also, and vegetation stops during 
the drought, to be resumed with the wet season. 
An almost universal feature in xerophytes is a thick 
