CH. Il] PHOSPHORUS METHOD. 41 



(48 A) Farmer's experiment 1 . 



Mount two Elodea leaves in water and seal the 

 preparation with wax-mixture. Place it in the dark for 

 3 or 4 hours or until the circulation ceases for want of 

 oxygen. Then expose it to incandescent-gas light when 

 the circulation will start in a few minutes. 



(49) BoussingauWs phosphorus method. 



A glass tube, in form like a test-tube 2'5 cm. diameter 

 and 20 25 cm. in length is fitted with a good rubber cork. 

 On the inside of the cork fix a stiff wire which will serve 

 to support about a centimeter of phosphorus. Push half 

 way up the tube a mass of water plants, e.g. Ranun- 

 culus aquatilis or Potamogeton sp. Fill the tube with 

 water and partially replace the water by hydrogen to 

 which about 5 per cent, of C0 2 is added. The cork 

 carrying the phosphorus is then inserted, and the tube 

 is placed in the dark. The white fumes which appear 

 will subside in an hour or two, and the atmosphere in 

 the tube will be clear. It is then placed in sunshine, 

 when the oxygen evolved will again form an obvious 

 white cloud. 



(50) Pfeffers method*. 



A leaf is exposed to light in a calibrated tube con- 

 taining a known volume of CO 2 : after a certain number 

 of hours the amount of C0 2 decomposed is estimated by 

 absorbing what remains with KHO. The tube is almost 

 36 cm. in length, of which 26 cm. is a calibrated tube of 



1 Farmer, Annals of Botany, Vol. x. F. Darwin, Proc. Cambridge 

 Phil. Soc. Vol. rx. 



2 Sachs' Arbeiten, i. p. 15. See also Pfeffer's Physiologic, i. p. 188. 



