NUTRITION 367 



by pressure, showing that they are of a soft, elastic, and semi-fluid con- 

 sistency. 



Pigment and stroma. — In fact, the body or stroma of the chloroplasts 

 seems to be like the cytoplasm, but dyed by the green pigment. The 

 precise relation between the pigment and the stroma has not been satis- 

 factorily made out, even in the killed chloroplast, and in the live un- 

 altered chloroplasts it can only be conjectured. In some cases, when 

 the pigment has been dissolved out by alcohol, the stroma (of course 

 coagulated by the alcohol) presents a spongy appearance, and it has 

 been inferred that the meshes of the sponge throughout were occupied by 

 pigment. In others, especially in the larger chloroplasts which can be 

 sectioned, the pigment seems to be restricted to a spongy shell of measur- 

 able thickness at the surface, while the interior is colorless. 



Pigments. — The yellow-green pigment is called chlorophyll; but 

 it is not a single substance. Several pigments can be separated more or 

 less completely, of which only two are abundant and constant in all higher 

 plants, the one bluish green and the other pale yellow. The names 

 applied to these are confusing. To distinguish them we shall employ 

 the terms chlorophyllin and carotin. To the bluish green one no dis- 

 tinctive term has been generally applied, but it has been usually called 

 chlorophyll (not distinguishing it from the combination), or chlorophyll 

 proper. For the yellowish one, xanthophyll, ct'wlin, and carotin have 

 been used. The last is preferable. 



The term xanthophyll is descriptive, but it has also been used for other minor 

 yellow pigments. Etiolin was applied to the pale yellow pigment which appears 

 when plants have been " etiolated " by being grown or kept for a time in darkness. 

 It seems to be identical with the yellow pigment named from the carrot, carotin, 

 which proves to be very widely distributed in plants. 



Chlorophyllin and carotin may be partially separated by their unequal 

 solubilities. If to a fresh solution of chlorophyll in 80 per cent alcohol, 

 benzene be added, the mixture shaken, and then allowed to stand, the ben- 

 zene rises, carrying the greater part of the chlorophyllin, while the alco- 

 hol retains the greater part of the carotin. 



Chlorophyllin. — The chemical composition of chlorophyllin is not 

 known. It is very easily altered and is certainly very complex, contain- 

 ing N as well as C, H, and O. Whether phosphorus or magnesium is 

 an essential constituent is in contention. Iron does not seem to be an 

 integral part of it, though considered essential to its formation. The 

 red coloring matter of the blood, hemoglobin, yields decomposition prod- 



