38 BRITISH PLANTS 
with this statement, but such plants are exposed to the 
same danger in the physiologically-dry winter. Rosette- 
plants have often long tap-roots which are perennial, 
and the stunted stems are known as root-stocks. 
A third form of stunted growth is the cushion-growth, 
a habit assumed by many plants growing in alpine situa- 
tions. The stems do not elongate, but they branch 
freely close to the ground, forming dense cushions (Silene 
acaulis, Saxifraga hypnordes). 
2. Leaf-Modifications.—The leaf, because of its stomata, 
is the chief transpiring organ of the plant. Any reduction 
of the leaf-surface entails a diminution in the number 
of the stomata, and consequently a reduction in the 
amount of water transpired. On the other hand, the 
green leaf is conspicuously the seat of food-construction, 
and therefore any diminution in the loss of water can 
only be effected at the expense of assimilation. If the 
leaves are small, less food is made, growth is checked, 
and the whole plant suffers. Reduced assimilation, 
however, is the lesser of two evils, and most small-leaved 
xerophytes show manifest signs of impaired nutrition ; 
in fact, the diminution in the size of the leaves is evidence 
in itself that the plant is imperfectly nourished. 
The effect of a xerophytic environment is expressed 
in the size, form, characters, and display of the leaves 
more than upon any other vegetative organ. The leaf 
is essentially an expression of its environment. Where 
moisture is abundant, and there is no danger of desiccation, 
the leaf is generally large and thin. As the environ- 
ment becomes physiologically drier, the leaf tends to 
exhibit one or more of the characters enumerated below. 
This does not mean that the removal of any particular 
plant to drier surroundings will result in any correspond- 
ing modification of its leaves. The size, form, and 
characters of leaves are, within narrow limits, fixed for 
every species. What is really meant is that those plants 
whose leaves display xerophytic characters, do so because 
they are best adapted for dry situations, and such plants 
will consequently be found there to the exclusion of all 
other plants not so well equipped to contend with the 
perils of drought. 
The various forms and characters associated with the 
leaves of xerophytes may be looked upon as the outcome 
