206 Movement of Water in a Robinia. 



An examination of the tables will show that the first action of 

 the stump wasthat of suction, — the mercury, which had risen at the 

 moment the tube was drawn over the stem, having quickly fallen. 

 This we were led to expect from the rapid transpiration of the shoot, 

 and the consequent exhaustion of moisture from the stem before it 

 was cut. This suction continued and reached its maximum at the 

 hottest part of the day, or when the parent tree was most actively 

 transpiring, evidently pulling upon the water in the manometer 

 through the root-stock of the sucker. After the maximum was 

 reached, the action decreased until sunset, when a slight pressure 

 was observed ; and. if the records had been made during the night, 

 it is probable that a still higher pressure would have been registered. 

 In the second table, it is noticeable that during the rain of the 

 morning, there was recorded only one third the amount of negative 

 pressure that appeared at 3 p.m., when the air was warm and the 

 sun shining. At the close of the experiment, the water in the 

 rubber tube was replaced by air from the stem, and all fluctuations 

 after that time were evidently caused, at least in part, by varia- 

 tions in the temperature acting upon the column of air above the 

 stump. 



The experiment is interesting as showing how vigorously a 

 transpiring tree draws upon its roots even to the distance of 30 

 ft., and upon its young shoots, in opposition, to the more feeble 

 draught of the sucker, as the moisture evaporates from its leaves. 

 These stems wilt very quickly when cut down, showing, as did the 

 manometric action, that the stems were almost empty. 



From the fact that air in some cases finally replaced the water, 

 it appears that, in spite of all precautions to the contrary, the appa- 

 ratus, while air-tight for slight pressures, did not prove so for those 

 of considerable increase ; so that, the maximum of negative pressure 

 permitted by the apparatus being reached, the mercury then fell 

 back to a height which was within the limits of the instrument. 



So far as we are enabled to determine from these results, it 

 would thus appear : — 



1st. That the influence of transpiration is felt in very remote 

 parts of the plant. 



2nd. That, in this case at least, root-pressure has but little value 

 in supplying the wants created by transpiration. 



It will be the object of further experiment to see how the roots 



