INFLUENCE OF ENVIRONMENT ON PLANTS 337 



lie immediately under the epidermis of a small depression 

 of the surface, and which communicate with the exterior 

 by a few fine pores which perforate the latter. The 

 epidermis of this depression is made up of cells with thin 

 non-cuticularised walls. Each so-called gland is in contact 

 with the end of a nbro-vascular bundle, whose sheath is 

 carried forward over the general mass of delicate cells 

 (fig. 143). The depression of the surface is filled with a 

 mass of carbonate of lime, which is originally excreted by 

 the leaf, and which is held in its place by a few papilla? 

 which project from the epidermis. Such an arrangement 

 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 fibro- 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 



22 



