CHAP. VII ATMOSPHERIC PRECIPITATIONS 31 



can accomplish this, for instance, by employing hairs or the velamen of 

 aerial roots to condense the vapour. Possibly the cases in which such 

 absorption has been assumed may be due to the deposit of liquid water 

 taking place in or on plant-parts as a consequence of change in tempera- 

 ture. The fact that plants drooping on a warm day become turgescent 

 at night, does not imply that water-vapour has been condensed from the 

 damper night air, but is certainly to be ascribed to the circumstances 

 that transpiration is decreased owing to the smaller saturation-deficit, 

 and that the roots, which may have been continuously conveying water 

 to the plant, supply at night more water than is transpired. 



Certain desert-plants excrete hygroscopic salts which at night-time 

 abstract water from the moister air ; but it is not certain that this water, 

 which moistens the surface of the plant, is absorbed and utihzed by the 

 cells.^ 



Atmospheric precipitations. 



If from any cause the air be cooled to dew-point, so that it cannot hold 

 in a vaporous state the water which it contains, the water is deposited 

 in one of the three known forms of atmospheric precipitation, mist (clouds), 

 rain (snow), or dew (hoar-frost). Atmospheric precipitations in part are 

 absorbed by the soil and thus become a source of profit to the plant,^ and 

 in part are retained by epigeous portions of the plant,^ with which they 

 come into direct contact and which in certain cases seem to be adapted 

 to absorb them. Many plants (epiphytes and hthophytes) have no source 

 of water other than direct atmospheric precipitations. 



Adaptation for the absorption of atmospheric precipitations} In order 

 that a plant may absorb atmospheric precipitations it is necessary that 

 the cell-wall shall be permeable, the superficial cells shall contain osmo- 

 tically active substances, and that the water shall not flow off the surface 

 too rapidly. There are plants such as lichens, mosses, and certain algae, 

 which can absorb liquid precipitations easily and rapidly over the whole 

 surface, and thus become turgescent ; these plants also endure extreme 

 desiccation. Other plants have on the surface definite parts which can 

 be wetted and absorb water, but have other parts which do not permit 

 this, or, at all events, can be wetted with difficulty (owing to thick cuticle, 

 coating of wax, and the like). For the purpose of absorbing water from 

 atmospheric precipitations a number of plants have special organs (aerial 

 roots with peculiar absorbing tissue ; spongy old plant-remains that 

 greedily suck in water ; hairs, like those in Bromehaceae, capable of 

 taking up water ; characteristic leaf-cells with perforated walls, and so 

 forth. ^ 



But it must be assumed that water is absorbed by epigeous organs, as 

 a rule, only when these are in a half-faded condition, and when the root 

 can provide no water and the plant contains no reserve-supply ; absorption 

 by the root is a matter of necessity to the normal land-plant.^ 



' Volkens, 1887; Marloth, 1887 ; J. Schmidt, 1904; Massart, 1898 a. 

 ' See Chapter XII. ' Sec Burgcrstein, \(:)Oa. 



* See subsequent Sections, especially those dealing with xcroph>-tes (Chap. XXXI, ) 

 ' See subsequent Sections, especially those dealing \\'ith xerophytcs. 

 " Bohm, 1863; Detmer, 1877; Tschaplowitz, 1892; Kny, 1895; Willc, 18S7; see 

 Burgcrstein, 1904. 



