WATER UTILIZATION BY TREES 



hi 



The opponents of this view base their opinion for the most part on 

 work done with herbaceous plants; and it may be that in these smaller 

 forms diffusion processes are quite sufficient to satisfy the metabolic 

 needs without the use of the transpiration stream. Haberlandt {80), 

 pointing to the luxuriance of tropical vegetation, calls attention to the 

 fact that here, where many of the largest trees are found, the tran- 

 spiration rate is at a minimum and cites this as proof of the fact that a 

 rapid transpiration stream is not essential for the growth of luxuriant 

 vegetation. While he admits that the transpiration stream may help 

 somewhat in the transport of nutrients, he believes that ifc is not the 

 most important agent. The supporters of the functional hypothesis, 

 on the other hand, call attention to the fact that while the transpiration 

 stream in tropical trees is not rapid, it is very continuous and during 

 the lifetime of the plant enormous amounts of water pass through in 

 this fashion. 



One does not need, however, to turn to tropical vegetation to find 

 abundant proof of the relation between transpiration and mineral 

 content. Stahl (204) reports that in Populus tremula, where the 

 motion of the leaves increases the transpiration rate, there is also an 

 increase in the mineral content of the leaves. 



Ebermayer (56) even felt so sure of the relation between transpira- 

 tion and the mineral consumption that he believed the water demands 

 can be deduced from the ash content. In support of these views he 

 found that the leaves of plants which transpire more rapidly, such as 

 ashes, willows, alders, maples, and elms, contained the most ash (7 to 

 10 percent) ; while hornbeam, oak, and beech leaves showed only 4 to 5 

 percent; silver fir, spruce, and larch 2.9 to 3.5 percent; and forms de- 

 manding very little water, such as Austrian pine and Scotch pine, 1.3 to 

 2 percent. 



Huber (104), in his work on differences in transpiration with differ- 

 ences in level in Sequoia, found a distinct parallelism between the de- 

 crease in transpiration rate and decrease in ash content in twigs from 

 increasing heights. Sample data are given in table 25. The figures 

 from the twig at 10 m do not seem to fit in with the others, but it is 

 noteworthy that both the transpiration and the ash content in that 

 particular twig were exceptionally high. 



Table 25. — Relation between- transpiration rate (based on fresh weight) and ash 

 content (based on dry weight) of Sequoia twigs at different elevations above the 

 ground 



Elevation (meters) 



Transpira- 

 tion rate 



Ash 

 content 



Elevation (meters) 



Transpira- 

 tion rate 



Ash 

 content 



2 .. 



Percent 



ion 



91 

 69 



68 



Percent 



100 

 82 

 72 

 74 



10 



12 



14 



Percent 

 91 

 57 

 53 



Percent 



90 





70 



6 



85 



8 











Burns (29), in work in Vermont with white pine seedlings, noted 

 that those seedlings 8 to 10 weeks old which grew in full sunlight tran- 

 spired 21 times as much as those in full shade, while those in half shade 

 transpired 8 times as much as those in full shade. The dry- weights 

 per average of 1,000 seedlings and the ash percentages (based on dry 

 weight) were as shown in table 26. 



