36 BRITISH PLANTS 



Xerophytic Characters exhibited by Plants living under 

 Physiologically Dry Conditions. 



The difficulty with xerophytes is to retain within the 

 tissues sufficient water for their needs. If, from any 

 cause, little is absorbed, then little must be lost. To 

 secure this, the organs on which are found the exits for 

 the escape of water are modified. 



Thus leaves and shoots are modified in form and 

 structure, curious and characteristic habits of growth 

 are assumed, and the display of the leaves to the light 

 is not the same as in ordinary plants. 



Whatever the means adopted, the end is always the 

 same to keep down transpiration to a minimum, in order 

 that as much water as possible may be retained within 

 the body of the plant. 



1. Stunted Growth of Stems. This is brought about 

 by lack of nourishment, a condition experienced by all 

 xerophytes whose water-supply is limited. Trees become 

 stunted and dwarfed, assuming the low bush-form in 

 regions where the xerophytic conditions become pro- 

 nounced e.g., in semi-deserts, on dry, windy plateaux, 

 in cold alpine regions, and near the limits of tree-growth 

 towards the Pole. Under extreme conditions, the trees 

 may be only a few inches high, and the annual output 

 of leaves not more than two or three. On the crests of 

 Snowdon, the common juniper forms a great branching 

 mat, lying prostrate on the rocks. The soil is thin, and 

 the plants are exposed to violent desiccating winds, great 

 heat at noon, severe cold at night; and intense illumina- 

 tion when the sky is clear. The internodes are short 

 and the development of buds is feeble and irregular. 

 The mat-like growth serves to keep the plant just out of 

 reach of the most violent wind, and at the same time 

 keeps the soil underneath it shady and moist. 



This form of stunted growth is not, however, permanent, 

 because under more genial conditions many stunted alpines 

 will develop tall stems, while, on the other hand, plants 

 which are several feet high on the plains shrink to a few 

 inches on high alps and wind-swept downs (see p. 77). 



Although light has some effect in dwarfing plants by 

 promoting transpiration, it has a direct effect upon 

 growth, which is far more important. Shoots and leaves 



