ABSORPTION OF FLUIDS AND SOLIDS BY SUB-AERIAL ORGANS 161 



A simple experiment of Boussingault's suffices to demonstrate that salts may 

 pass through the cuticle. If a drop of a very dilute solution of KNO,, Ca SO &c. 

 is placed on a beech or other leaf, tiny crystals form on this spot when rapid 

 evaporation is allowed, but not when the leaf is covered with a watch-glass, so that 

 the drop evaporates slowly and the salt has time to penetrate. Hence, small 

 amounts of dissolved substances carried down by rain or dew may be directly 

 absorbed by the foliage leaves. For further details see the literature referred to, 

 in which will be found discussions as to whether hairs and other organs are adapted 

 to retain water or lead it away. It may be mentioned that frequently the cuticle 

 over the veins is especially permeable to water. 



Condensation of water vapour. All plants must be supplied with 

 fluid water, for even in air completely saturated with water-vapour full 

 turgidity cannot be restored to a flaccid tissue. Pfeffer found that a moss 

 (Catharinea nnditlatd] had not absorbed the full amount of water necessary 

 to produce normal turgidity, after being kept at a constant temperature 

 for twelve days in a saturated atmosphere, in which the cell-walls soon 

 swelled and hence caused the shrivelled leaf to expand. If, however, 

 a formation of dew is induced by changes of temperature, complete 

 turgidity may be attained in from two to four days. This result was 

 obtained both with plants rooted in a little soil and with others suspended 

 freely, for the soil only becomes sufficiently moist when dew is formed 

 in abundance. The formation of dew which takes place in ordinary 

 plant-houses does not suffice for this, for freely hanging epiphytic orchids 

 gradually decrease in weight, if left unwatered 1 . 



Air-dried plants are, however, able to condense a certain amount of 

 water when placed in air saturated with moisture, and this is indicated by 

 the twisting of the beaks of Erodiwn gniinmn, or of the awns of Stipa 

 pennata, which are used as hygrometers, and by the increased pliability 

 of the thallus of Laminaria or of Lichens. This condensation of water- 

 vapour in the substance of the cell-wall is only possible by means of the 

 enormous surface-tension energy brought to bear (Sect. 12), and as the 

 amount of water absorbed increases, this force rapidly decreases, so that 

 further absorption takes place more and more slowly. R. Hartig found 

 that wood-shavings kept at a constant temperature in saturated air were 

 still increasing in weight after fifty-seven days. Sachs found that the 

 maximal swelling was attained much more rapidly, but this was probably 



Beit rage zu Biol., 1887, Bd. IV, p. 310; Chimielewsky, Hot. Centralbl., 1889, Bd. xxxvm, p. 790; 

 Ganong, Bot. Centralbl., 1894, Bd. i.ix, p. 180. On the absorption of water by water-pores, see 

 Habcrlandt, Sitzungsb. d. Wiener Akad., 1894, Bd. cm, Abth. i, p. 502 ; 1895, Bd. civ, Abth. i, 

 pp. 96, no. Cf. also Sect. 48. 



1 Unger, Sitzungsb. d. Wien. Akad., 1864, Bd. XXV, p. 179; Duchartre, Compt. rend., 1869, 

 T. LXVii.p. 773. 



PFEFFER M 



