120 



BOTANY OF THE LIVING PLANT 



materially increased, though so far there has been no extensive application 

 of this knowledge to practical operations. Ultimately a point is reached at 

 which the gas begins to exert a harmful effect on the plants, though previous 

 to this it may be observed that continued addition of carbon dioxide produces 

 little further benefit ; probably some other factor such as light intensity is 

 inadequate to support higher rates of photosynthesis, and it now becomes 

 the limiting factor. 



It is by virtue of the evolution of oxygen during photosynthesis that 

 plants can be said to " improve the air " in the presence of light, a 



Fig. 80. 

 Arrangement for showing that oxygen is given off in Photosynthesis. 

 A, before exposure to light, the tube is filled with water. B, after 

 exposure for some time. A large volume of discharged gas has 

 collected in the tube. (After Noll.) 



property which was noticed many years ago. The oxygen that is pro- 

 duced in photosynthesis diffuses from the chloroplasts through the 

 cytoplasm and cell-walls and thence in the gaseous form through the 

 air-spaces of the leaf, and finally emerges into the atmosphere, apart 

 from that utilised in respiration (see p. 133). The production of the 

 gas is most easily demonstrated in water-plants. If a cut shoot of 

 Canadian Pond Weed (Elodea) lying in water is exposed to bright 

 sunlight, a continuous stream of bubbles may be seen to escape from 

 the cut end of the shoot, especially if the water has previously been 

 enriched with extra carbon dioxide. The temperature also must be 

 favourable. The gas can be collected by the arrangement shown in 

 Fig. 8o, and tests will show that the gas contains a high proportion of 



