ABSORPTION-CELLS ON LEAVES. 237 



It is also worthy of mention that papillae are developed near the absorption- 

 cells, with a view to the retention of the salt crystals, similar to those which hold 

 the calcareous incrustations on the leaves of saxifrages and Acantholimon. The 

 leaves of plants covered with crystals of salt are also for the most part furnished 

 with little bristles, to which the salt adheres so firmly that it is not readily detached, 

 even by violent shaking. 



But however striking the analogy may be between the development and 

 significance of lime crusts and salt crusts, there is the essential difference that the 

 former have not, like the latter, the power of attracting moisture from the air. 

 And on this particular stress must be laid. In the broken and hillocky tracts 

 on the shores of salt- lakes or of the sea, where tamarisks and frankenias are 

 especially wont to live, the sandy ground dries up to such an extent in the height 

 of summer that it is scarcely conceivable how plants growing in it are able to 

 preserve their vitality. The proximity of the sea has no immediate efiect on the 

 moisture of the ground in such situations. The sea- water does not penetrate into 

 the ground far beyond the high-water line, and it is out of the question that the 

 layers of soil serving as substratum to the frankenias and tamarisks should be 

 irrigated by subterranean water. When in summer there is an absence of rain for 

 months together, these plants — even though in close proximity to the sea — would 

 necessarily perish of drought. Only the circumstance that they turn to account the 

 moisture of the atmosphere by means of the excreted salts renders it possible for 

 them to flourish in these most inhospitable of all inhospitable sites. 



Many plants which are periodically exposed to great dryness have the tips of the 

 teeth on the leaf -margins thickened into little cones or warts. They also glitter 

 somewhat and at times are sticky. The glitter and viscidity are due to a resinous 

 slimy substance, which often contains sugar and tastes sweet. This substance 

 covers the teeth and sometimes spreads from the teeth inwards to a great dis- 

 tance over the face of the leaf in the form of a delicate film-like varnish. The 

 greatest resemblance exists between this varnish (sometimes known as " balsam ") 

 and the secretions of the glands on the leaves of the Alpine Rose and of the 

 glandular hairs on those of Centaurea Balsaviita. It is excreted by special cells, 

 which are intercalated in the epidermis of the foliar teeth, and are at once marked 

 out from the other cells of the epidermis by the facts that their protoplasm is of a 

 brownish colour and that their external walls are easily permeable by water. The 

 excretion of the varnish-like layer takes place at a time when the entire plant is dis- 

 tended with sap, chiefly, therefore, in the spring. When summer is at its height 

 the varnish dries and thenceforward affords an excellent preservative from the risk 

 of too much evaporation from the cells it covers, and especially from those situated 

 on the teeth of the leaves by which it was excreted. But if this dried film of 

 varnish is wetted it saturates itself quickly with water and renders moisture 

 accessible to the cells beneath it. Thus its value is similar to that of the crusts of 

 lime and salt on the leaves of the plants above described. When moist it eflfects 

 the absorption of water, when dry it guards against desiccation. 



