CHAP. XXX TRANSPIRATION IN LAND-PLANTS 103 



and Henslow * suggest that the arrangement serves to scatter and weaken 

 the incident rays of hght. Haberlandt,- on the contrary, regards these 

 lenticular cells as organs for the perception of light. 



Wax may be excreted over the surface and depress transpiration, as 

 has been experimentally established by Tschirch ^ and Haberlandt. 

 Usually the coating produced is only a thin one ; but, to take an opposite 

 case, Capparis spinosa, at the commencement of the dry season in the 

 Egyptian desert, produces over the whole leaf-surface a very thick layer 

 of wax that completely prevents transpiration.* The coating of wax 

 may be very thick, more than one millimetre in Sarcocaulon in South 

 Africa, and up to five milhmetres in Wax Palms. Incrustations of wax 

 cause plant-members to have a dull, matt, bluish surface, which is then 

 said to be covered with ' bloom '. Such ' bloom '-covered leaves usually 

 have at their margins no sharp teeth, and possess, at most, rounded teeth 

 provided with hydathodes. Wax prevents water from wetting leaves, 

 so that it protects ombrophilous fohage from rain.^ 



Incrustatio7is of salt are produced on the surface of some desert-plants, 

 w^hich thereby acquire a grey tint and are perhaps protected against 

 excessive transpiration ; at night the incrustations dehquesce as they 

 absorb moisture from the atmosphere.^ In the Plumbaginaceae and 

 certain species of Saxifraga the hydathodes which excrete calcic carbonate 

 may possibly ser\'e to check transpiration, but their main function would 

 seem to be the excretion of injurious salts. 



Varnished leaves. Resins or similar bodies are excreted by hairs on 

 the surface of many, particularly austral, xerophytes. The leaves are 

 thus rendered viscid and appear as if lacquered, since they acquire a 

 glossy, vitreous investment ; the epidermal walls are thin and feebly 

 cutinized.' 



The creosote bush (Larrea tridentata) in the North-American deserts 

 has leaves which, when unfolded, are thin, but which gradually become 

 coated with shellac.^ 



Mucilage, or a mixture of gum, resin, and other bodies, is sometimes 

 excreted by hairs (colleters^), in the buds of Polygonaceae and others: 

 it may possibly aid in the absorption of water and perhaps check transpira- 

 tion during the flushing of the foliage. 



The contents of epidermal cells may be designed to depress transpira- 

 tion. The epidermis is perhaps a water-reservoir ^^ when, as is usually 

 the case with land-plants, it is colourless. In virtue of various substances 

 contained by it the epidermis may become less permeable to water- 

 vapour. Tannin is often markedly present in the epidermis of ever- 

 green leaves during winter,^^ and appears in connexion with the aqueous 

 tissue of desert-plants and steppe-plants such as Alhagi, Monsonia, 

 Astragalus, Tamarix ^^ ; but the functional significance of these facts is 

 obscure. 



Anthocyan is a red pigment present in many plants, and particularly 

 in the epidermis ; according to Engelmann and Stahl ^^ it acts as a heat- 



' Henslow, 1894. - Haberlandt, 1905. * Tschirch, 1882. 



* Volkens, 1887. ' Burgerstcin, 1904, p. 207. " Compare pp. 31-2. 



' Volkens, 1890. ' Coville, 1893, p. 51. " Hanstcin, 1868. 



'" Westermaier, 1882. " Warming. 1884. 



" Volkens, 1887 ; Henslow, 1894. " Engelmann, 1887 ; Stahl, 1896. 



