ABSORPTION OF GASES. 



41 



If the double-walled bell-jars are not at hand, each may be re- 

 placed by two glass cylinders, one so much larger Fig. 38. 

 than the other that when the smaller is fastened in- 

 side the other by means of a cork, a space of about 

 I to 2 cm. remains between them. Fill this space 

 with the proper solution, and the inner vessel with 

 water containing carbon dioxide, and place in the 

 latter the plant-shoots. To cut off the perpendicu- 

 lar rays cover the apparatus with a loosely-fitting 

 cardboard cover. (Fig. 37.) The pasteboard box 

 shown in Fig. 35 can also be used if instead of the 

 ground-glass plates (a a) parallel-walled glass cells 

 filled with the absorption fluid are used. With colored Apparatus to' 

 glass plates only an approximately pure light can be demonstrate 

 obtained. If the box is used, it will be found most of oxygen, 

 convenient to place the shoot in an inverted test-tube (Mangin.) 

 filled with water as in Fig. 38. The amount of gas can be measured 

 directly in the tube. 



32. Product of Photosynthesis. — The product of photosyn- 

 thesis is probably some soluble carbohydrate such as glucose. 

 As soon as enough of this substance has been formed to meet 

 the immediate needs of the plant the remainder is converted 

 into starch. If the plant is placed in darkness or under any 

 condition in which it cannot carry on photosynthesis, as soon 

 as the glucose in the cells is consumed the starch is recon- 

 verted into glucose or some form of sugar and assimilated. 

 The amount of starch present in a plant may be taken as 

 an indirect indication of the amount of photosynthesis. 



EXPERIMENT 47. 



MACROCHEMICAL TEST FOR STARCH. 



Boil a few leaves of Bean, Tomato, or Tropaeolum for a few 

 minutes to kill the protoplasm and swell the starch-grains present. 

 Place in warm alcohol until the chlorophyll is dissolved. Bring 

 the leaves into an alcoholic solution of iodine for a half-hour. 

 The leaves will be colored a dark blue if they contain starch. 



