280 TRANSFER OF WATER THROUGH THE PLANT. 



has been shown b}- many investigators, notabl3- by Sachs, ^ who 

 found that well-rooted and full-leaved plants of gonrd and 

 tobacco wilted when the temperature of the air and soil ranged 

 from 3.7° to 5° C, although the ground was plentifully supplied 

 with water. When the temperature of the soil became higher, 

 the leaves became again turgescent. 



Another cause which may disturb the relation between absorp- 

 tion and transpiration is found in the diminished conductivity of 

 wood}' tissue at low temperatures.^ 



749. Checks upon transpiration. Among the more obvious adap- 

 tations of plants to dry climates are : (1) reduction of foliage to 

 a minimum, as in the case of condensed stems (see Vol. I. p. 64) ; 

 (2) a coriaceous or even denser texture of leaves or of branches 

 resembling leaves, such as phyllocladia (Vol. I. p. 65) ; (3) ver- 

 tically placed leaves or their analogues, phjllodia, in msmy if 

 not most of which the sti'uctnre of the parenchyma and of the 

 ei^idermis with its stomata is the same on both sides ; hence 

 the sides have substantially the same exposure to air, and, in 

 the compass leaves, to light as well (see 448). Another adap- 

 tation has been pointed out bj- Pfitzer^ and bj- Westermaier ; * 

 namely, the possession of an epidermal or subepidermal "water 

 tissue," or '■ water-storing tissue " (see 209). 



Leaves provided with water-storing tissue show the effect of 

 drought first in the partial collapse of these cells, their radial 

 walls becoming somewhat undulate, while the assimilating cells 

 remain full and unchanged in foi'm. These water-storing cells 

 lose comparatively little water by transpiration ; the water which 

 they contain is given up as required to the assimilating paren- 

 chyma. When a fi'csh supply of water is afforded to the 

 collapsed water-stoi-ing tissue, the recovery of tnrgescence is 

 immediate. Examples are found in the following among many 

 other plants : Peperdmia, Tradescantia discolor, Fiens elastica. 



In numerous succulents the vacuoles of the assimilating cells 

 frequently contain a thin mucus, from which water evaporates 

 only slowly, and this is believed to play an important part in 

 the storage of water.^ 



1 Botanisclie Zeltung, 1860, p. 124. 



2 Beitrage z\u- Theorie des "Wiuzcklruclces, 1877, p. 38, quoted by Tfeffer, 

 Pfianzenphysiologie. 



^ Ueberdie melii'schichtige Epidermis, Pringsheim's Jahrb.,viii., 1872, p. 16. 



^ Uebei- Ban uiid Function des pflanzliclien Hautgewebesystems, ibid., xiv. 



* Plants whicli are peculiarly adapted to dry climates are termed by 



De CandoUe Xerophilous. Among them are found many CompositiE,, notable 



