70 WATER KXUDED. [CH. Ill 



SO as to be parallel to the two others, is tied into a piece 

 of pressure tube which is also tied to the plant. The arm 

 B passes through a rubber cork firmly tied into a wide- 

 mouthed (stoppered) bottle, in the bottom of which is half 

 an inch of mercury, Hg : the tube M, which serves for 

 manometer readings, fits tightly into a hole in the cork 

 and reaches the bottom of the bottle. Water W is now 

 poured in at G so that the bottle and the arm T are filled. 

 At first the plant will usually absorb water, so that G 

 should be left open until the rise begins, when it may be 

 filled up and closed by means of a clamp. The mercury 

 will rise to a considerable height and will show diurnal 

 variations about its mean position which should be 

 carefully noted. 



(90) Moll's Experiment. 



Various kinds of plants, when placed under a bell- 

 jar standing in a dish of water, will give evidence of 

 root pressure by the drops of water exuding from the 

 leaves. Root pressure may as Moll has shown^ be replaced 

 by that of a column of mercury. The branch or leaf-stalk, 

 as the case may be, is fixed air-tight into the short arm of 

 a U tube filled with water, and mercury is then poured 

 into the long arm until about 20 cm. pressure is obtained. 

 The whole is then covered with a bell-jar standing in 

 water^ and after a time drops of fluid are found hanging 

 to the leaves. We found that with 25 cm. of mercury 

 the drops appear very rapidly on the leaves of the 



1 Bot. Zeitvng, 1880, p. 49. Eeferences are given to Saehs' Lehrbuch, 

 1874, p. 660, and de Bary, Bot. Zeitung, 1869, p. 883, for similar results. 



