CH. l] SUCCULENTS. 9 



soaked in water before they are used, but placed air-dry 

 on wet filter paper in the flask. 



The mercury rises in the tube in spite of the absence 

 of KHO. As a control experiment an equal weight of 

 barley should be placed in precisely similar conditions. 

 In our experiments the column of mercury was depressed 

 in the case of the barley. 



The two flasks should be kept as nearly as possible 

 at a constant temperature ; for an accurate determination 

 of the results, barometer readings must be taken at the 

 beginning and end of the experiment, and corresponding 

 corrections must be made. But any external influences 

 will affect the experimental and the control flask equally, 

 so that the difference between their readings must depend 

 on the different behaviour of the seeds in germination. 



(11) Succulents^. 



In certain succulents an increase of the acidity of the 

 cell sap is accompanied by a fixation of oxygen. There- 

 fore, when the plant is placed in the apparatus used in 

 experiment 10, a rise of the mercury takes place. The 

 following is taken from Detmer, p. 224. 



He recommends Rochea falcata as especially good for 

 the experiment. A leaf is taken from the plant at the 

 close of a hot summer day, cut into pieces and introduced 

 into the eudiometer (Detmer, fig. 14). The lower end of 

 the graduated tube is placed in water and the apparatus 

 kept in the dark until the following morning when a 

 considerable rise in the water column is visible. As a 



^ See De Saussure, Becherches Ghimique. (An. xii=1804), p. 65. 



