306 Mr. A. J. Wilmott. Experimental Researches on 



bubbles may not exceed 2 per cent, for several hours. Bubblers delivering 

 even as slowly as one bubble in 8'5 minutes gave constant results to within 

 2 seconds. 



One very great advantage of these bubblers is that the same one may be 

 used for different plants in succession, and so strict comparisons made inde- 

 pendently of the size of the plant's own capillary openings. 



Unless the bubblers are quite clean, the bubbles may tend to cling to the 

 tip. This can be cured by boiling tliem in a test-tube with distilled water. 



With the comparatively large slow bubbles of the glass bubbler, it is easy 

 to see that the range of natural rates does not affect the size of the bubbles. 

 One never gets an almost continuous stream of very tiny hubbies, but, after 

 each escapes, the water springs back a little way into the capillary nozzle- 

 and remains stationary for a bit while the internal pressure gets up ; then it 

 is quickly pushed out to give a nearly full-sized bubble, after which the 

 bubble slowly increases to full size against the surface tension till the 

 critical point is reached and it breaks away. This pulsating rhythm is 

 repeated with great steadiness. 



Ac. Variations of Size of Bubbles due to Varying Surface- Tension of the, 

 Solution. — A line of research, for which the bubble method has been used by 

 Treboux, Jacobi, Pantanelli and others, is the study of the effect, on the 

 rate of photosynthesis, of substances added to the water so as to penetrate 

 into the plant. Any alteration of surface-tension of the water by these 

 solutes must directly affect the size of the bubbles given off, so that bubble- 

 counting ceases to give an accurate measure of the change produced in the 

 assimilating cells. This complication can be completely avoided by the use 

 of another simple device. The bubbles are in our experiments delivered not 

 into the body of the experimental liquid, but into a small cup which 

 contains distilled water, so that the tension is unaffected when solutions are 

 changed. This cup is shown, together with the bubbler, in fig. 1. It consists 

 of a glass tube 18 mm. wide, closed below by a cork, which has a hole^ 

 through which the inverted plant stem is pushed, and secured above and 

 below with a tenacious wax mixture of low melting point. The shoot is 

 first attached to the cork and bubbler, and then the outer tube is slipped 

 down on to the cork to form the bubbling cup. 



Ad. Variations of Size of Bubbles due to Osmotic Action of Liquids on the 

 Stem Aperture. — Pantanelli, Treboux and others have set out to investigate 

 the effect of quite strong solutions of various salts upon photosynthesis in 

 water plants. It is clear that, without special precautions, such solutions- 

 must shrink the cells bounding the stem aperture, and so enlarge the 

 aperture, and probably the size of the bubbles. 



