WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 181 



into such a position that bubbles begin to come off, and 

 count the rate ; when it becomes fairly constant, bring the 

 lamp to half this distance from the plant and count again. 

 Part of the effect, however, is due to the heat given out by 

 the lamp ; a flat- sided bottle vessel of water (kept cold by 

 constant renewal) should be used as a screen to absorb the 

 heat. 



240. Non-starchy Leaves. If a variety of plants be 

 tested for starch-formation by photosynthesis, it is found 

 that in many cases the amount of starch present, even 

 under the most favourable conditions, is small, while others 

 produce no starch at all. Most non- starchy leaves produce 

 relatively large quantities of sugar, and there is in general 

 an inverse ratio between the amounts of sugar and of starch 

 produced by the leaves of different plants. 



On the afternoon of a warm bright day collect leaves of 

 Onion and of Sunflower (or other starch-leaved plant). 

 (1) Test an Onion leaf with iodine ; no starch is formed by 

 this plant. Cut a second Onion leaf into pieces and boil 

 with Fehling's solution ; abundant sugar is present, as 

 shown by the copper oxide precipitated. (2) With a meat- 

 juice press crush separately the chopped-up leaves of the 

 two plants ; in each case measure the volume of juice 

 obtained, boil it, let it cool, replace the water lost in boil- 

 ing, and filter. Now determine the volumes of juice 

 required for the reduction of 20 c.c. of Fehling's solution ; 

 a very small amount of Onion juice is sufficient, but a much 

 larger amount of Sunflower juice is required. 



Keep an Onion plant in darkness for two days ; cut off 

 portions of several leaves, noting either their fresh weight 

 or their total length ; press out the juice, and determine 

 the volume of juice required to reduce 20 c.c. of Fehling. 

 Now expose the plant to light for several hours ; cut off an 

 equal fresh weight, or total length, of leaves, press out the 

 juice, and determine its reducing power with Fehling as 

 before. The result will show that the Onion leaf pro- 

 duces sugar by photosynthesis. Now place the plant in 

 darkness again, and after a day test the reducing power 

 of the juice ; the sugar content of the leaf is diminished, 



