Movement of Water in a Robinia. 203 



of Aroidese, where transpiration is comparatively weak, it 

 may even exceed the evaporation from the leaves, and cause 

 exudation in drops from their tips. This is by no means 

 the rule, however ; on the contrary, the loss caused by transpira- 

 tion usually tends to produce a vacuum in the stems and lower 

 tissues, which is only slowly compensated for by root-pressure. 

 This is shown by the well-known fact that if an actively 

 transpiring plant be cut off a few centimetres above the ground 

 and water applied to its cut surface, it is rapidly sucked in, and it 

 is only after the lapse of some time that root-pressure predomi- 

 nates, but then water is forced up the stem and out at the cut 

 surface in large amounts and even against pressure. This action 

 will continue until the roots die, with a force, varying but gener- 

 ally increasing to the maximum, and then gradually decreasing 

 until the force is exhausted. 



It was to determine the movement of water, chiefly as exhibit- 

 ed in root-pressure under the influence of atmospheric conditions, 

 that the following experiments were entered upon. 



A mercurial manometer of the ordinary form, nine decimeters 

 long and with graduations reading to mms., was set up in the open 

 air beside the plant, and connection with the stump was made by 

 a rubber tube bound with book-linen to prevent expansion under 

 pressure. All the fastenings were made air-tight, and all air was 

 removed from the water column connecting the mercurial column 

 with the stump, and the tube was then pressed down over the stem, 

 which was cut off four cm. from the ground, i.e., below the lowest 

 leaf. The plants selected were vigorous shoots of Robinia pseudaca- 

 cia of this year's growth. All were from 6 to 10 deem, in height 

 0.5 and 0.7 cm. in diameter, and were situated from 15 to 20 ft. 

 from the parent trees. An examination showed an intimate con- 

 nection between the suckers and these trees. The running roots 

 from the tree give rise to buds which develop into strong shoots 

 having at first no root system of their own, the old root continu- 

 ing its course and becoming the primary root of the new plant 

 before the connection with the parent tree is severed. All the 

 roots showed a swelling at the juncture with the sucker, and 

 some of them exhibited signs of decay a few inches back of 

 the swelling, but in most, the connection with the main root of the 

 tree was perfect, as the results of the experiment will show. 



