38 BRITISH PLANTS 



with this statement, but such plants are exposed to the 

 same danger in the physiologicaUy-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 hypnoides). 



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 

 perUs of drought. 



The various forms and characters associated with the 

 leaves of xerophytes may be looked upon as the outcome 



