WATER UTILIZATION BY TREES 55 



Oliver (165, pp. 26-28), using a potometer, placed twigs of Acer 

 pseudo plotanus in containers in which various amounts of S0 2 were 

 introduced. At a concentration of 1 part S0 2 to 13,300 of air (i. e., 

 0.6 mg of S0 2 per cubic foot) there was a sudden depression of the 

 water absorption, and at a concentration of 1 part in 6,700 the tran- 

 spiration was very considerably lowered. 



Although Y\ r ieler (236), using twigs of various woody plants, and 

 Stoklasa (211), using small trees of Picea excelsa, did not find any 

 depression in transpiration through the influence of SQ 2 , their results 

 are quite contrary to those already mentioned and also to the 

 results of Neger and Lakon (161), who repeated (apparently more 

 carefully) the experiments of Wieler and came to the conclusion that 

 S0 2 hindered photosynthesis and was directly toxic to the plant. 

 Also, when the transpiration of healthy trees (Acer platanoides, 

 Picea excelsa, Pinus strobus) was compared with that of trees that 

 had been damaged by S0 2 , both the water intake and the transpira- 

 tion were found to be lowered in the latter. Their conclusion, there- 

 fore, is that under the influence of S0 2 , as soon as the assimilation 

 apparatus has been injured, a decrease in water absorption occurs 

 which is often much greater than the decrease in transpiration, with 

 the result that the leaves dry up. 



Schroder and Reuss (184) found that a leafy branch of Acer plat- 

 anoides which was not subject to S0 2 absorbed 26 g of water and gave 

 off 33.05 g, while a twig which had stood for 6 hours in an atmosphere 

 containing 1 part of S0 2 in 1,000 of air absorbed 12.6 g and gave off 

 8.15 g; the latter thus absorbed about one-half as much water as the 

 healthy twig and transpired only about one-fourth of the amount. 

 Neger and Lakon, who carefully copied this experiment, found, in 

 general agreement with Schroder and Reuss, that the water absorp- 

 tion and transpiration of twigs exposed to S0 2 were much less than 

 those of healthy twigs; but in both twigs there was a small increase 

 in weight, which they explain by the fact that the experiment was 

 performed under bell jars where transpiration was decreased. Accord- 

 ing to Schroder and Reuss, only the twig exposed to S0 2 increased in 

 weight, but if that were actually the case, Neger and Lakon point 

 out it would be hard to explain why the leaves exposed to S0 2 in the 

 open should so soon have the appearance of dried leaves. The decrease 

 in transpiration brought about by S0 2 is probably explained as due 

 to injury to the guard cells and the consequent closing of the stomata, 

 although this point has not been thoroughly cleared up. 



POSITION OF LEAVES ON THE STEM 



Certain factors which influence transpiration can hardly be classed 

 as either internal or external but are really a combination of the two. 

 In this category may be placed the position of the leaves on the stem. 

 Lower leaves not only have different internal physiological relations 

 from the upper ones but they are also placed in a different environ- 

 ment. Huber (104) has studied this question more intensely than 

 anyone else. Twigs were cut off at various heights (2, 4, 6, 8, 10, 12, 

 11m) from a tree of Sequoia washingtoniana (Wins.) Sudw. 15 m high 

 and then placed in the same environment (not in water) and weighed 

 at hourly intervals. In all cases there was a decided falling off in the 

 transpiration rate with increasing stem level, so that the transpiration 

 of the lowest twigs was, in certain cases, six times as great as that of 



