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, etiolin, 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 yeHow 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 
Ted color; ng matter of the blood, hemoglobin, yields decomposition prod- 
