250 BOTANY PART i 



them to absorb the light. If their function of C0 2 assimilation is 

 to be well performed the foliage leaves must not only have a large 

 surface but also be thin. Practically it appears that light which has 

 passed through one or two foliage leaves is unable to exert any further 

 assimilatory effect. The leaves must, however, contain a very large 

 number of chlorophyll grains. Their dark green colour shows that 

 this is the case, and microscopical examination confirms this. Stems 

 have far fewer chloroplasts than the leaves, and the roots and other 

 subterranean organs have none at all. 



Every investigation shows that organs without chlorophyll are 

 quite unable to assimilate carbon dioxide. This holds not only for 

 the organs of the plant but for the parts of the cell. The colourless 

 protoplasm and the nucleus of the cell give off no oxygen when 

 exposed to sunlight ; this can readily be proved by the bacterial 

 method (p. 248). The chloroplasts alone are the active organs in C0 2 

 assimilation, and only when they contain chlorophyll ; etiolated or 

 chlorotic chloroplasts are not functional. 



In the red-leaved varieties of green plants, such as the Purple Beech and Red 

 Cabbage, chlorophyll is developed in the same mariner as in the green parent 

 species, but it is hidden from view by a red colouring matter in the epidermis or 

 in deeper-lying cells. In the Red Algae, on the other hand, the chromatophores 

 themselves have a red colour ; after death a red pigment (phycoerythrin) becomes 

 free, leaving the chloroplasts green. Regarding the pigments in the Phaeophyceae 

 and the Diatomeae cf. p. 19. 



In studying the effect of different kinds of light upon assimilation, it is custom- 

 ary either to use the separate colours of the solar spectrum, or to imitate them by 

 means of coloured glass or coloured solutions. SCHOTT and others have employed 

 red and blue glasses or double-walled bell-jars filled with suitably-coloured 

 solutions. 



Only a relatively small percentage of the light which falls on the 

 leaf and is absorbed is utilised in the assimilation of C0 2 ( 29 ). That, 

 however, light must disappear as such in C0 2 assimilation is clear, for 

 from what other source than the energy of light could the energy be 

 obtained that is stored up in the organic substance formed in assimila- 

 tion ? This potential energy of the organic substance of the plant 

 serves to maintain the vital processes. The force exerted by our 

 steam-engines is also to be traced to the assimilatory activity of the 

 plants, the wood or the carbonised remains of which are burnt 

 beneath its boiler. In the combustion of the reduced carbon com- 

 pounds to carbon dioxide the energy, which was previously required 

 to transform carbon dioxide into the combustible materials, again 

 becomes free. 



The assimilatory activity of a chloroplast, like every vital function, 

 is dependent on a number of internal and external factors. To the 

 internal factors belong the presence of the pigment chlorophyll and 

 its situation in a living chloroplast. Chlorophyll itself, separated from 



