METABOLIC PROCESSES IX THE PLANT. 331 



the exposure to the higher temperature, in spite of the exclusion 

 of light, has brought about a diminution in the acidity. 



Towards evening we take a pair of leaves from a plant of 

 Roehea (we may also experiment with Kcheveria or Bryophyllum). 

 In one leaf the acidity is determined at once, the other is longi- 

 tudinally halved. Each half, after being weighed, is cut up 

 into small pieces about 1 cm. long, and these are placed in retorts 

 filled with distilled water which has been boiled and then allowed 

 to thoroughly cool again. The water in one retort is replaced 

 by air, that in the other by pure Hydrogen (for method see 10). 

 Next morning we make determinations of acidity, and it is found 

 that the pieces exposed to air have produced much free acid, 

 while in those exposed to Hydrogen very small quantities at most 

 of free acid have been formed. Free Oxygen is therefore neces- 

 sary for copious production of acid. 



It has already been mentioned that access of light accelerates 

 the decomposition of organic acids outside the organism, a fact 

 which, as I previously remarked, is most certainly of interest 

 in connection with the question which we are here considering. 

 We can easily demonstrate (even in lecture) the influence of 

 light on the decomposition of acids. We fill a test-tube to the 



N 



top with a solution of Oxalic acid, add to it some freshly 

 5 



prepared Ferric hydrate (prepared by mixing Ferric chloride 

 solution with ammonia and carefully washing the precipitate), 

 and now invert the test-tube over mercury and expose the 

 solution, which, after some time, becomes intensely yellow in 

 colour, to direct sunlight. An evolution of gas at once begins;- 

 the gas (Carbon, dioxide) collects in the upper part of the tube, 

 while the fluid becomes colourless, and a precipitate of Ferrous 

 oxalate separates out. It is not impossible that the oxidation 

 of organic acids in the plant is similarly greatly promoted by 

 the direct influence of rays of light. 



Finally, we will make experiments which teach that succulent 

 structures, as already mentioned, actually take up a comparatively 

 large quantity of Oxygen, when acids are accumulating in their 

 cells. For the purpose of lecture demonstration it is quite 

 sufficient to proceed as follows : We take a leaf from a plant 

 of Rochea falcata on the evening of a hot summer day (the leaf 

 which I used weighed 24 gr.), cut it up into pieces, and place 

 the pieces in the upper expanded part of the eudiometer shown 



