NUTRITION, STORAGE, AND RESPIRATION 



101 



direct sunlight than in diffused hght : that red, orange, and yellow 

 rays of the spectrum are more effective than those of the blue or 

 violet ; and that the process itself depends upon the presence of 

 carbon dioxide dissolved in the water. Thus, if water be used from 

 which the carbon dioxide has been driven off by boiling, the bubbles 

 of oxygen will not be discharged. 



Fig. 72,- 

 Arrangeiuent for showing oxygen given off in Photo-Synthesis. A , before e.xposure 

 to light, the tube is filled with water. B, after exposure for some tunc. A large 

 Volume of dischai-ged gas has collected in the tube. 



The relation of the activity of Photo-Synthesis to temperature has been 

 illustrated by observations on the leaves of Hottonia, as reckoned in the 

 number of gas-bubbles given off in five minutes, during which period the 

 temperature remained constant. At 14° C. the number was about 200. 

 When the temperature was raised to 31' C. the optimum number of about 

 550 was reached, while at 56.5° C. the activity ceased. 



The effect of light in Photo-Synthesis is strictly local, tliat is, it is 

 limited to those points where the rays actually fall. This is sliown 

 by experiments with stencil plates. Letters or other figures are cut out 

 of an opaque card or tinfoil ; or a plain strip will serve. It is applied 

 closely to the surface of a green leaf of a living plant, which has pre- 

 viously been kept in the dark for two days, so that the starch will hav^^ 

 been completely removed from the cells (see p. 1 1 1). The leaf should 

 then be exposed to sunlight for six hours or so. If it then be removed, 

 boiled for a minute in water, decolourised with alcohol, and stained with 



