162 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



owing to a formation of dew, for which trifling differences of temperature 

 suffice l . 



Similarly salts deliquesce in saturated air, although dilute solutions, 

 such as the cell-sap, condense water-vapour with extreme slowness. Plants 

 of Reamuria hirtclla, Tamarix mannifera, &c., are covered with a saline 

 incrustation during the day-time, but nevertheless the condensation of 

 water-vapour is certainly insufficient to supply all the water the plant 

 requires. The hygroscopic saline incrustations of many desert plants may 

 induce an abundant formation of dew, but it has yet to be discovered 

 what are the precise means by which they make use of the water thus 

 obtained, for in order to draw water from such concentrated solutions, 

 the cell-sap would apparently need to attain a still higher osmotic con- 

 centration. It has yet to be determined whether this is attained by 

 regulatory adaptation, or whether perhaps the impermeability of the 

 cuticle is such as to afford a protection against highly plasmolytic saline 

 solutions -. 



Since the presence of salt lowers vapour-tension, water-vapour must necessarily 

 be transferred from pure water to a condensing saline solution, when both are 

 enclosed in a chamber filled with saturated air. Such distillation takes place 

 extremely slowly, for the potential differences which originate it are but slight. 

 Thus at 20 C., the presence of i per cent, of KNO S lowers the vapour-tension 

 of water only by about o-i mm. 3 In the plant, the attractions for water which 

 the cell-wall and cell-sap exert always set themselves in equilibrium, and hence 

 the cell-walls are in that condition of imbibition which lowers the vapour-tension 

 of water by the same amount as does the particular degree of concentration of the 

 cell-sap 4 . The osmotic energy of a solution, and hence also of a plant, may 

 accordingly be determined by means of its vapour tension (Sect. 24). 



If a i per cent. KNO S solution is raised from 20 C. to 20-2 C., the depression 



1 R. Hartig, Unters. a. d. Forstbot. Inst. z. Miinchen, 1882, Bd. II, p. 17; Sachs, Arb. aus 

 \Vtirzburg, 1879, Bd. n, p. 309. In other observations upon the absorption of moisture from the 

 air, sufficient care has not been taken to exclude all dew-formation. Wilhelm, Bot. Jahresb., 1883, 

 p. 39 (seeds) ; Detmer, Beitrage z. Theorie d. Wurzeldruckes, 1877, p. 18 (lichens) ; Kerner, Pflanzen- 

 leben, 1887, Bd. I, p. 201 (lichens and mosses); Borzi, Acqua in rapporto alia vegetazione, &c., 

 Auszug a. d. Acten d. internat. bot. Congresses, 1892. See also H. Dixon and J. Joly, Phil. Trans., 

 1895, vol. clxxxvi, p. 575. 



3 Volkens, Flora d. agypt. \Viiste, 1887, p. 27 ; Ber. d. Bot. Ges., 1887, p. 434 ; Marloth, ibid., 

 p. 321. See also Sect. 23. According to Marloth (1. c.), the saline incrustation contains 17-2 per 

 cent. NaNO 3 ; 12 per cent. MgSO7H,O; 5.5 per cent. NaCl; and 51.9 per cent. CaCO s . 

 Volkens' observation that the glands are not plasmolyzed is of little importance (Ber. d. Bot. 

 Ges., 1887, p. 434). Plasmolysis may be prevented by various means ; cf. Pfeffer, Druck u. 

 Arbeitsleistung, 1894, p. 307. In regard to nectaries see Sect. 49. 



3 See Ostwald, Allgem. Chemie, 1891, 2. Anfl., Bd. I, p. 707. 



* See Pfeffer, Studien zur Knergetik, 1892, p. 258. On swelling, see Sect. 12. Maxwell 

 (Theorie der Warme, 1878, p. 326) has shown how and why the concave menisci in capillaries lower 

 the vapour tension, and cause a condensation of water vapour; a capillary of o-ooi mm. bore 

 lowers the vapour tension by about one-thousandth part. 



