344 VEGETABLE PHYSIOLOGY 



upon rocks have leaves which show special structures for 

 absorbing water from rain or dew. Several plants, among 

 which some species of Saxifrage are conspicuous, possess 

 a number of glandular structures upon the teeth of their 

 thick narrow leaves. Each consists of a small mass of 

 cells with delicate walls, which 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 fibro-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 papillae 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 condense 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 



