RELATION OF TRANSPIRATION TO ABSORPTION. 279 



appreciabl}' ; if the plant is then kept at rest, the rate falls be- 

 low that previous to the shaking, after which it gradually rises 

 to its normal point. Even a sharp single shock is enough to 

 produce some effect upon transpiration, but the shaking must 

 continue at least a second in order to change the rate very much. 

 If, however, the shaking is long continued, or short shakings 

 are often repeated, there is a noticeable diminution in the rate. 

 Baranetzky attributes the heightening of the rate by a sudden 

 shock to the correspondingly sudden compression of the inter- 

 cellular spaces and the consequent renewal of the air therein 

 contained ; while the diminished rate which follows continued 

 shaking is due to a partial closing of the stomata (see also 

 731). 



746. Relation of age of leaves to transpiration. According to 

 Deherain 1 and Hohnel, 2 young leaves exhale more water than 

 older leaves. Experiments were made by the former upon the 

 upper, middle, and lower leaves of rye. From the newly devel- 

 oped leaves more water was exhaled than from the middle, and 

 more from the latter than from those farther down the stem. 

 Sachs 8 states that young leaves exhale less than those which 

 are fully developed, but that there is some diminution in the 

 case of old leaves. 



747. Under external conditions which are as nearly uniform as 

 can be secured there are variations in the rate of transpiration 

 not yet understood ; these are generally referred to variations in 

 the tension of tissues (see 1025). 



748. Be&^pn of transpiration to absorption. It is plain that 

 transpiration from leaves is the chief cause of absorption by 

 the roots ; but it has been shown by Vesque 4 that these two 

 functions are not necessarily proportional. According to him 

 it is only when a plant is subjected to uniform conditions of 

 diffused light, and a moderate amount of moisture in the air, 

 that they are about equal. In a very dry air, transpiration in 

 the case of most plants far exceeds absorption until wilting 

 comes on. When, on the other hand, a plant is withdrawn from 

 a moderately moist air and placed in an atmosphere saturated 

 with moisture, absorption goes on for a time more rapidly than 

 transpiration, but both become soon arrested. 



The dependence of the rate of absorption upon temperature 



1 Cours de Chimie Agricole, 1873, p. 178. 



2 Forschungen auf d. Geb. d. Agrikulturphysik, 1878. 

 8 Handbuch der Experimental-physiologie, 1865, p. 22 

 * Annales des Sc. nat., ser. 6, tome vi., 1878, p. 222. 



