216 BOTANY paut i 



assimilatoiy 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. 214). The chloroplasts alone are the active organs in COg- 

 assimilation, and every member of the plant becomes, caeteris parihis, 

 more active in assimilation the greater the number of chloroplasts it 

 contains. On this account the foliage leaf may be regarded 



AS the organ of CO2-ASSIMILATION. 



In the red-leaved varieties of greeu jjlants, such as the Purple Beech and Red 

 Cabbage, chlorophyll is developed in the same manner 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 this becomes free as a bluish-red pigment 

 (phycoerythrin) leaving the chloroplasts green. The Brown Algae and the 

 Diatomaceae have a brown modification of chloro})hyll (phaeopliyll) which after 

 death changes into chlorophyll (cf. p. 62). 



In the blue-green freshwater Algae, and also in the Brown and Red Seaweeds, 

 the maximum assimilation takes place, according to Engelmann, in another part 

 of the spectrum than it does in the case of green plants. The assimilation in 

 these Algae seems indeed to be carried on in the part of the spectrum the colour of 

 which is complementary to their own. According to Stahl the pigment of green 

 plants composed of a yellow and a green constituent is complementary to the 

 predominant rays of sunlight, which undergoes essential modification in its passage 

 through the atmosphere {^^). 



In studying the elfect 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. For such experiments it will be 

 found convenient to make use of double-walled bell-jars filled with a solution of 

 bichromate of potassium or of ammoniacal copper oxide. Plants grown under jars 

 filled with the first solution, which allows only the red, orange, and yellow rays 

 to pass through, assimilate almost as actively as in white light. Under the jars 

 containing the second solution, which readily pernuts the passage of tlie chemically 

 active rays, assimilation is much less active. 



Very little is known with regard to the processes carried on in the cliloroplasts 

 during assimihition, and it is still by no means clear what part the green chloropliyll 

 pigment performs. The jiigment, which may be extracted from the protoplasm of 

 tlie chlorophyll bodies, makes up only a small part of their substance (about 0"1 per 

 cent), and gives no reaction from which its o])erations may be inferred. The light 

 absorbed by the chlorophyll pigment also stands in no recognisable relation to the 

 requirements of assimilation, for the assimilation is not proportional to tlie intensity 

 of the absorption of the ditl'erent rays. 



Tlie proportion of the energy, passing through the leaf in the form of light. 



