1885.] On the transpiration-stream in cut branches. 331 



It will suffice to say that the two chief theories are 



i. That the chief and essential path of the stream is in the 

 walls of the elements of the wood ; that the stream is 

 in fact the movement of the water of imbibition con- 

 tained in the walls of the elements. 



This view we shall refer to as the "imbibition theory". 



ii. That the chief and essential path for the passage of 

 the stream is in the cavities of the elements of the 

 wood. 



The chief object of our experiments has been to help to decide 

 the question which of these theories is the right one. 



In the present paper we have not attempted to do more than 

 to test the validity of the two theories as applied to cut branches, 

 without considering whether or no the conclusions arrived at are 

 applicable to rooted plants. The method employed may, it is be- 

 lieved, be used with plants grown in water. 



§ 2. Method. 



The experiments here recorded were chiefly made with the 

 potometer already briefly described by one of us*. The instrument 

 is intended to measure the amount of water absorbed by a tran- 

 spiring plant in a given time, and has been called the potometer 

 in imitation of the name suggested by Moll*f* for a different instru- 

 ment. 



There is nothing new in the principle of our potometer, the 

 chief merit that we may claim for it is its convenience for working 

 purposes. 



Fig. 1 shows the potometer in action. It consists of a T tube, 

 of which the limb a is bent so as to be parallel to the remaining 

 two limbs, and into which a cut branch of a plant is fixed by an 

 india-rubber tube. The two other limbs of the tube are closed by 

 india-rubber corks, through one of which passes a thermometer tube 

 6. The T tube and the thermometer tube being filled with water 

 the apparatus is fixed so that the end of b dips into a small vessel 

 of water, c. It is obvious that all the water which the branch 

 absorbs must be obtained from the vessel c. To take a reading 

 with the instrument all that is necessary is to remove the block of 

 wood d so that the vessel c can be lowered ; when this is done air 

 instead of water will be sucked in at the end of b. When a 



* Francis Darwin in Nature, May 1, 1884. 

 t Archives Neerlandaises xviii. 188-4. 



VOL. V. PT. V. '2'S 



