174 WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 



under the mouth of the test-tube and transfer the latter, 

 still inverted, to a vessel containing potassium pyrogallate, 

 freshly made ( 233). Note what proportion of the col- 

 lected gas is absorbed by the pyrogallate, and therefore 

 consists of oxygen ; the gas not absorbed is nitrogen, but 

 that the collected gas is richer in oxygen than ordinary air 

 is shown by the fact that more than one-fifth of it is ab- 

 sorbed by the pyrogallate. 



232. That Atmospheric Carbon Dioxide is essential 

 for Photosynthesis is readily proved. The most satis- 

 factory method is to analyse the air in which plants have 

 been confined under conditions favourable for photosyn- 

 thesis. This direct analysis method is difficult because of 

 the small amount of carbon dioxide concerned, but con- 

 vincing results are given by the indirect method of placing 

 similar green leaves in light under conditions exactly alike 

 except that atmospheric carbon dioxide is allowed access 

 in one case and is excluded in the other, and noting whether 

 starch is produced. 



Another method is to deprive water-plants of carbon 

 dioxide by boiling the water (and thus driving off all 

 dissolved gases) before placing the plants in it and ex- 

 posing them to the light, and noting whether oxygen is 

 given off. 



(a) A rough method of excluding air, and therefore 

 atmospheric carbon dioxide, from the leaf is to treat 

 starch-free leaves of Fuchsia, Tropaeolum, or other hypo- 

 stomatal (with stomata confined to lower surface) leaves 

 as follows. Smear different leaves with vaseline (a) on 

 the lower side only ; (fc) on the upper side only ; (c) on 

 both sides ; (d) on a band-like area of both sides. These 

 experiments are not, however, conclusive, since the smear- 

 ing necessarily interferes with respiration and transpira- 

 tion. 



(6) Tie the stalks of starch-free Tropaeolum leaves to a 

 stone, and sink them under water in a large jar ; expose 

 to light for several hours, then test with iodine. A little 

 starch may be formed, since the leaves of land-plants are 



