LOSS OF WATER 249 



in as fast as it is lost if there is an abundant supply, but since 

 there would be no need for this replacement if it were not lost 

 by transpiration, it is hard to see how this can in any sense be 

 called a " function." 



The effect upon salts is a more disputable one. It was once held 

 that the transpiration stream swept along with it the salts in 

 solution in the soil water and thus aided in their circulation through 

 the plant. Although it would take many years for a molecule of 

 salt to reach the top of a giant sequoia by simple diffusion, after 

 the water gets into the vessels of the wood, there are comparatively 

 few cell walls to pass through before it gets to the living cells of 

 the leaf, and the transpiration stream certainly helps to further 

 the rapid circulation in this open stream; but since the passage 

 from the soil into the vessels depends upon osmosis and the gen- 

 eral laws of diffusion, it is difficult to see how transpiration could 

 increase directly the rate of entrance from the soil, owing to the 

 fact that salts and water move independently according to their 

 own partial pressures. 



Schlosing (1869) grew tobacco under a bell jar and also free in 

 the air. In the moist atmosphere under the bell jar, the ash was 

 13% of the dry weight, while out in the dry air, where there was 

 plenty of evaporation, the ash was 22% of the dry weight. This 

 would indicate that transpiration does increase the amount of 

 salt taken in by the plant. Mendiola (1922) failed to confirm 

 the results of Schlosing, and McLean found that in tropical rain 

 forests the shade leaves, with a lower transpiration rate, had a 

 higher mineral content than the sun leaves; but Muenscher (1922) 

 found that when transpiration of barley was checked by lowering 

 the illumination, the ash content was decreased. Likewise Haas 

 found that in windy weather, when the transpiration of Citrus 

 was increased, there was an increase of calcium salts in the leaves. 

 If the transpiration stream hastens the movement of salts within 

 the vessels it would seem only to be expected that this would re- 

 move the salt from the base of the plant near the roots, and ac- 

 cording to the laws of diffusion more would then diffuse in. The 

 accumulations of salt are known to be higher in the leaves than 

 in any other plant organs (Chap. X), and this accumulation and 

 storage in the leaves would then entail an entrance of more salts 

 below. It is hence the conclusion of the writer that while it is not 

 the function of transpiration to bring in more salts from the soil, 



