PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 313 



coast. Plants of steppes and prairies {e.g. Centaurea Balsamita of the Persian 

 steppes and Grindelia squarrosa in the prairies of North America) are likewise 

 protected throughout life from over- vaporization by varnish-like coverings of this 

 kind; while the foliage of Cherry, Apricot, and Peach trees, as well as of Birches, 

 Sweet Willows, Balsam and Pyramidal Poplars, and the Black Alder, is only covered 

 with such a varnish while young, when it has just burst from the buds, and the 

 outer walls of the epidermal cells have not yet become sufficiently thickened; later 

 on, however, when the cuticularized layers have become fully formed, this covering 

 which limits transpiration disappears. Only on those places of the epidermis, where 

 the outer walls of the cells remain very thin and permeable by fluids and gases, is 

 this coat of balsam retained until the leaf is to be thrown ofi"; but in this case it 

 probably regulates the absorption of atmospheric w^ater. 



How far the incrustations of lime and salt excretions take part in the absorption 

 of atmospheric water by organs situated above the ground has likewise already been 

 considered in the section on water absorption. It is obvious that these concretions 

 and coverings of the epidermis must be capable of restricting transpiration. 

 Incrustations of lime are principally found in plants which grow^ in the clefts and 

 crevices of rocks; excretions of salt are only observed in shore-plants and those of 

 steppes and wastes, but then always on low bushes and shrubs with small narrow 

 leaves, and herbs whose foliage rests on the soil. The reason for this is again easily 

 found. High trees could not support the weight of leaves loaded with incrustations 

 of lime and salt, even if their trunks and branches possessed the greatest strength 

 imaginable. 



It has been observed that plants whose leaves are covered by incrustations of 

 lime and salt, or whose epidermal cells are strongly thickened on their outer walls by 

 corky layers, are almost always destitute of hairs; while plants, on the other hand, 

 whose epidermal cells possess delicate outer walls, if they are not surrounded 

 by a damp atmosphere throughout the year, nor submerged in water, are usually 

 furnished with structures known as plant-hairs (trichomes); from which it may be 

 inferred that the hairy covering of the leaf or stalk in question is able to protect it 

 from drying up in just the same way as the corky layers. Of course only those 

 hairs are meant whose protoplasmic contents have disappeared, and which have 

 become sapless and filled with air; for those hair-structures, which consist of cells 

 rich in sap and osmotic contents, would not help in preventing evaporation from the 

 deeper tissue ; they are themselves in need of protection, and special protective 

 arrangements exist for them, as already set forth in the discussion on the absorption 

 of water by aerial portions of the plant. Such structures would, if unprotected, 

 give off water to the surrounding air, and continually absorb fluid from adjacent 

 cells below them. This action does not take place in air-containing cells, and if their 

 dry membranes, and the air which they inclose, are interpolatetl between the dry 

 atmosphere and the succulent tissue below, this latter wall be protected from evapo- 

 ration, like damp earth covered with a layer of dry straw or reeds, or the fluid at 

 the bottom of a bottle whose neck is closed with a plug of cotton-wool. 



