276 TRANSFER OF WATER THROUGH THE PLANT. 



a plant exposed to it dimiuishes.^ When the air is completely 

 saturated, a slight amount of transpiration can take place,^ 

 wiiich, as Sachs has pointed out,' is probably due to the fact 

 that the temperature of the plant is higher than that of the 

 surrounding air. 



740. Instructive experiments upon the exhalation of moisture 

 by some of the more common desert plants in the dry air of the 

 Western plains have been made by Sereno Watson,* from which 

 it appears that in about four hours young shoots furnished with 

 about fifty per cent of leaves lost, when severed from the stem, 

 water amounting to nearly half their weight. 



741. Effect of the soil upon transpiration. The physical prop- 

 erties of the soil have an influence upon transpiration. Sachs ^ 

 cultivated [ilants of tobacco in clay and in sandy soil, and ob- 

 served the amount of water transpired by them under like con- 

 ditions. Although his experiments are not conclusive, they 

 indicate that transpiration is more uniform from the foliage 

 of the plants grown in «lay-thaii from the plants grown in 

 sand ; the former soil is much more retentive of moisture, and 

 tlius the suppl}' of hygroscoi)ic water is given up more gradually 

 to the roots of the plant. 



The chemical properties of soils affect transpiration to a cer- 

 tain extent. Senebier, in 1800, stated that acids increase the 

 rate of transpiration, and he ascribed the same effect also to. 



1 The relations between humidity of the air and transpiration are shown 

 by tlie results obtained by Unger with two plants of Ricinus, one of which 

 was in the open air, the other under a bell-jar. (The leaf surface of one jilant 

 was X90, and that of the other 160 square centimeters ; but in the table a cor- 

 rection has been made so that equal surfaces are compared). 



The total losses bear a ratio of 10.44 : 1. 



2 Handbuch der Experimental-physiologie, 1865, p. 227. Deherain in 

 Comptes Eendus, l.xix. p. 381. 



3 Sitzungsber. d. Wiener Akad., Bd. xxvi., 1857, p. 326. 



* Report of the Geological Exploration of the Fortieth Parallel, Botany 

 (1871), p. 1. 



^ Versuchs-Stationen, 1859, p. 232. 



