36 PLANT ASSOCIATIONS 
If, however, we consider some of the formations or 
associations which are the result of extreme condi- 
tions of environment, we get more light on the rela- 
tions between the plants and the factors which 
control the vegetation. Take the case of the plants 
inhabiting desert regions such as were discussed in 
Chapter I. Here the outstanding feature is scarcity 
of water, and the plants display various remarkable 
adaptations which fit them for a thirsty life. There 
are three ways to meet scarcity of water—facilities 
for gathering it, arrangements for storing it, and 
economy in using it; and arrangements for all three 
are familiar features of desert plants. To effect the 
first, the root-system is extended, and is often 
enormously developed in proportion to the aerial 
parts. This adaptation may be studied in the flora 
of dry places in our own country, such as shingle 
beaches and sand dunes, which are characteristic semi- 
deserts. Take such plants as the Sea Holly (Eryngium 
maritimum), the Sea Convolvulus (C. Soldanella), or 
the Sea Sedge (Carex arenaria), and compare the ex- 
tent of the root-system or underground stems with 
that of the aboveground portions. Fig. 4 represents 
the Wild Carrot (Daucus Carota) as found growing 
under extreme exposure on the west coast of Ireland. 
To meet the conditions the tall branched stem has 
been entirely dispensed with, and the terminal umbel 
is seated on the ground in the middle of a ring of 
leaves. In this way the plant prepares to resist both 
drought and wind. Water storage is often developed 
in different parts of xerophytes (drought-resisting 
plants)—in roots, or stems, or leaves, which become 
much enlarged, and at the same time covered with a 
