338 



VEGETABLE PHYSIOLOGY 



serves a double purpose ; any dew or rain which reaches 

 the surface of the leaf is absorbed by the carbonate of lime 

 and can make its way slowly into the gland, whence it 

 passes into the nbro-vascular system ; while, when the leaf 

 is dry the incrusting mineral matter serves as a plug to 

 the depression, and reduces transpiration. 



Many plants which inhabit sandy deserts- possess 

 similar mechanisms ; some excrete carbonate of lime, 

 others crystalline accumulations of common salt. The 

 latter can not only absorb dew and rain but can also con- 

 dense and take up moisture from the air. They are found 

 occurring in such sandy wastes as are by the seashore or 

 near salt lakes. 



Many trees which grow in temperate climates, in poor 

 sandy soil on the margin of streams, show a somewhat 

 similar mechanism, but the excretion from their leaves 

 takes the form of a kind of resinous varnish or balsam 

 which can be readily wetted and which can absorb water. 

 In some cases so-called glandular hairs discharge a similar 

 function. 



The water which is absorbed in this way is rarely pure, 

 but contains traces of sulphuric acid and ammonia, which, 

 though trifling in amount, are no doubt of value in the 

 nutritive processes. The adaptation to their environ- 

 ment which these plants exhibit is thus chiefly in the 

 direction of economising a limited water supply. 



The influence of the environment on the form of the 

 plant can be seen equally well in the case of such plants 

 as grow in Alpine regions, where the cold is usually intense, 

 and the atmosphere for long periods so humid that transpira- 

 tion is only occasionally possible, and where consequently 

 the absorption of food materials is much impeded. Similar 

 conditions mark the bleak moorlands of temperate climates. 

 These show very great differences between the extremes 

 of temperature which mark summer and winter respectively. 

 The water supply also shows very great variations at 

 different times of the year. The plants are generally of 



