102 



be shown by means of the green water p^ant mentioned in the 

 previous experiment, for if it is placed in a cylinder of red 

 glass the bubbles will be evolved almost as quickly as in ordinary 

 sunlight, whereas in a cylinder of blue glass, which intercepts 

 and cuts off the red and orange rays, the evolution of bubbles 

 Light absorb- almost ceases. If the light which has passed through a 

 e< * by chlor solution of chlorophyll is decomposed by a prism, a dark band 

 will be found in the red and orange indicating that these 

 rays have been absorbed by the chlorophyll. The, latter, 

 the presence of which is essential for the assimilation of carbon 

 in the higher plants, is thus seen to absorb energy from the 

 sun in the shape of certain rays of light. Assimilation is, 

 however, a vital function performed by the living protoplasm 

 which, in some way which is not yet understood, utilises 

 the energy absorbed by the chlorophyll in decomposing the 

 Formation of wa -ter and carbon dioxide and in building up from them a carbo- 

 Starch. hydrate, such as starch. The chlorophyll is thus merely 

 a part of the apparatus which the protoplasm employs in this 

 work of assimilation. As might be expected, it 'will be found 

 that a leaf exposed to the light which has passed through 

 fi solution of chlorophyll, or through another green leaf, is 

 unable to assimilate carbon in the way described, the effective 

 light rays being no longer available for the process. 



Among the factors which directly influence assimilation 

 are a suitable supply of iron and carbon dioxide, the tem- 

 perature, and light, for each of which there is an optimum 

 degree of intensity. It has been already noted that no chloro- 

 phyll is formed unless the necessary quantity of iron is available 

 and that without carbon dioxide the assimilation of carbon 

 becomes impossible. The amount of carbon dioxide in the 

 air is as a rule very small, about "04 % and provided 

 that the light is sufficiently intense assimilation may be 

 increased by increasing the amount of carbon dioxide in the 

 air up to a certain extent. More than 10 % car- 

 bon dioxide, however, has a decidedly injurious effect 

 and many plants die if continually exposed to air containing 

 as little as 4 %. Temperature has a marked effect on assi- 

 milation, as well as on all other vital functions, and if the 

 temperature is below a certain minimum the protoplasm 

 forms no chlorophyll. In the absence of light assimilation 

 ceases at once, while it increases in proportion to the intensity 

 of the light until the optimum degree of intensity is reached, 

 after which it again decreases. If the intensity of the light 

 continues to increase the chlorophyll is ultimately destroyed. 



