198 BOTANY 



are equally capable of arousing the assimilatory activity. Just as the 

 rays of different refrangibility differ in their action, both upon the eye 

 and the photographic plate, so they have a different effect upon 

 assimilation. It would be natural to suppose that the chemically 

 active rays, the blue and violet, which decompose silver salts and other 

 chemical compounds, would also be the most effective in promoting 

 the assimilatory activity of the chlorophyll bodies. Exactly the 

 contrary, however, has been shown to be the case. The highly 

 refractive chemical rays have little or no effect on assimilation ; the 

 red, orange, and yellow rays, that is, the so-called illuminating rays of 

 the spectrum, are on the contrary the most active. 



In the blue-green fresh-water Algae, and also in the brown and red Seaweeds, 

 in which the chromatophores contain true chlorophyll in addition to their peculiar 

 special colouring matter, the maximum assimilation takes place, according to Ensel- 

 mann, 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. All the rays of the mixed 

 white light are usually at the disposal of plants growing freely in the open air ; 

 only the Seaweeds found in deep water (at the most but 200 m. deep) grow in a 

 prevailing blue light, while the deeper-lying tissues of land plants live in red light, 

 as this penetrates further into the parenchymatous tissues. 



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

 customary 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 oxides. 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 (90 per cent) as in white 

 light (100 per cent). Under the jars containing the second solution, which readily 

 permits the passage of the photo-chemical rays, assimilation is extremely low (5-7 

 per cent). 



But little is known with regard to the processes carried on in green cells during 

 assimilation ; and although it is evident that only the green chlorophyll bodies 

 are capable of assimilating, it is still by no means clear what part the green chloro- 

 phyll pigment performs. The pigment which may be extracted from the protoplasm 

 of the chlorophyll bodies makes up only a small part of their substance, and gives 

 no reaction from which its operations may be inferred. The light absorbed by the 

 chlorophyll pigment also stands in no recognisable relation to the requirements of 

 assimilation ; for the blue and violet rays, which are inoperative in assimilation, 

 are most strongly absorbed (see p. 59). It has not as yet been determined 

 what part the mineral constituents of the transpiration current take in the 

 process. On the other hand, the protoplasmic body of the chloroplasts cannot 

 assimilate when the green pigment is not present ; that is, when from any cause 

 the corpuscles are prevented from turning green. For, as the existence of the green 

 pigment is dependent upon the presence of iron, upon a proper temperature, and, 

 with few exceptions (Ferns, Conifers), upon the action of light, its formation in the 

 chlorophyll bodies may be prevented by depriving them of the requisites for its 

 development. The chromatophores will then remain yellow (in leaves) or white 

 (in stems), and no longer assimilate. 



