SECT, ii PHYSIOLOGY 217 



(phycocyan) leaving the chloroplasts green. The Brown Algae and the Diatomaceae 

 have a brown modification of chlorophyll (phaeophyll) which after death changes 

 into chlorophyll. 



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

 the maximum assimilation takes place, according to ENGELMANK, 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. The pigments associated with the 

 chlorophyll thus appear to act in the same way as the sensibilisators on the 

 photographic plate ; they attract light of a different wave length to co-operate 

 in the chemical process. 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 400 m. below the surface) grow in a prevailing blue light, 

 while the deeper-lying tissues of land plants live in red light, as this penetrates 

 farther into the parenchymatous tissues ( 37 ). 



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. 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 as in white light. Under the jars 

 containing the second solution, which readily permits the passage of the chemically 

 active rays, assimilation is much less active. 



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

 assimilation, and it is still by no means clear what part the green chlorophyll 

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

 the chlorophyll bodies makes up only a small part of their substance (about O'l per 

 cent.), 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 assimilation is not proportional to the intensity 

 of the absorption of the different rays. The proportion of the energy, passing 

 through the leaf in the form of light, utilised in assimilation is, according to the 

 thermo-electric measurements of DETLEFSEN, only 1 per cent ; according to the 

 calculations of H. BROWN, ^ per cent in sunlight, and over 2 per cent in diffuse 

 light f 38 ). 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 proto- 

 plasmic 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. As the existence of the green pigment is dependent upon the presence of 

 oxygen, of iron, of carbohydrates and other food substances, upon a proper temper- 

 ature, and, with few exceptions (Ferns, Conifers, cotyledons of Sycamore, lower 

 Algae in culture solutions), upon the action of light, its formation in the chloro- 

 phyll bodies may be prevented by depriving them of the requisites for its develop- 

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



Within recent years it has, indeed, been determined that certain nitrifying 

 bacteria have the power of forming a small amount of organic substances from 

 carbonates, carbonic acid, and ammonia. The process by which the organic 

 carbon compound is derived must, however, be altogether different from that of 

 green plants, as the bacteria contain no chlorophyll, and their nutritive activity 

 is in no way dependent upon the light. The necessary energy is here obtained 



