WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 193 



occupy 2 c.c., the volume allowed for them in the graduation of the 

 tube connected with the chamber ; this tube is joined by rubber 

 tubing to the reservoir tube, which has index marks 25 c.c. apart. 

 Pour mercury into the reservoir-tube until it stands level at the 

 100 c.c. mark in the graduated tube and at the lowest index mark 

 of the reservoir itself ; the stopper is inserted with its air-opening 

 matching that of the neck, and then twisted, so that the chamber 

 is sealed without compression of the air. 



After three or four days of growth in a shaded place, the reservoir 

 tube is slipped off under water, allowing the mercury to run out ; 

 then the rubber tube is clipped and the graduated tube slipped first 

 under potash and then under pyrogallate solution, to measure 

 the volume of carbon dioxide produced and that of any oxygen 

 left over in the apparatus, which originally contained 100 c.c. 

 of air. 



253. The Respiratory Equation. Weigh two sets, 

 each of ten seeds, of Oats or Barley. Soak one set (A) 

 for twelve hours and place it in the chamber of a respiro- 

 meter, with potash in the measuring-tube ; when the 

 potash stops rising, measure the volume of the carbon 

 dioxide. Dry the two sets of seeds, using the percentage 

 of moisture in B to determine the original dry weight of 

 A, from which calculate the loss of dry substance by 

 respiration in A. From the molecular weights in the 

 equation 



C 6 H 10 5 + 60 2 = 600, + 5H 2 



calculate the relation between the starch used up (loss of 

 dry weight) and the carbon dioxide formed, and see 

 whether they agree with or approximate to the formula. 



Since starch is converted into sugar in germination, 

 and sugar is usually the first complex product of photo- 

 synthesis, we may regard the respiratory equation as the 

 reverse of the photosynthetic equation and express the 

 two by the reversible equation 



C 6 H 12 O 6 + 6O 2 JT 6C0 2 + 6H 2 O. 



But it must be remembered that (1) this equation is 

 largely conventional ; (2) it expresses only the end result 

 and not the complex intermediate stages ; (3) the volumes 

 of gas absorbed and evolved in respiration are by no 

 means always equal, as the equation would imply. 



P. B. 13 



