208 MOVEMENTS AND EXCHANGES OF FLUIDS 



lowing apparatus has been designed by the author and tested by- 

 several years' use. A capillary tube is graduated into several 

 portions containing ioo mg. of water, and a length of i cm. at 

 one end is bent downward at right angles, while a double length 

 is bent into U form at the opposite end. To this U is attached 

 a 3 -way tube by means of rubber tubing wired. The upper end 

 of the 3-way tube is attached to a separatory funnel by means of 

 a rubber tube, or it may be fused on permanently. The long 

 calibrated tube is supported on a wooden base and the 3-way 

 tube by means of an iron post driven in the base. 



To determine the amount of water used by a shoot, fill the 

 separatory funnel and all of the tubes with water. Select a 

 fuchsia, geranium or any woody-stemmed plant and lay it in an 

 aquarium, or tank of water, and cut off the stem in such manner 

 that the negative pressure will carry water up into the vessels after 

 the excision is made (Fig. 106). It maybe necessary to allow the 

 shoot to stand in the water for a day before being fitted to the 

 apparatus. Fasten a section of rubber tubing to the free end of 

 the 3-way tube and wire it. Trim the base of the stem obliquely. 

 Now open the stopcock of the funnel and allow water to run 

 slowly out of the tube to which the branch is to be fitted. Insert 

 the base of the stem in the section of rubber tubing, taking care 

 that no air bubbles are included, and wire tightly. Clear the 

 tubes of all air bubbles, and set a small bottle under the end of 

 the capillary tube at B. Place a thermometer near the plant, and 

 have a watch convenient. Allow a small bubble of air to enter 

 the tube, and as the transpiring plant withdraws water from the 

 system of tubes, the bubble will traverse the capillary tube. Mark 

 the number of minutes, or seconds, necessary for the bubble to 

 travel through each section containing 100 mg. and when it has 

 passed the last calibration turn the stopcock, and force it back 

 beyond the first mark and repeat as often as desirable. 1 



The influence of negative pressure on such tests is illustrated 

 by the following experience. A leafy shoot of fuschia in which 



1 MacDougal. A convenient potometer. Bot. Gazette. 24: no. 1897. 



