THE COMPONENTS OF PLANTS AND ANIMALS 13 



18. Hemicelluloses. These polysaccharids differ from true 

 cellulose in being hydrolyzed by comparatively short boiling 

 with dilute acids and further in the fact that the hydrolysis, 

 instead of yielding only dextrose, as in the case of cellulose, 

 produces a variety of both hexose and pentose sugars, the 

 former including galactose, mannose and levulose, as well as 

 dextrose, and the latter arabinose and xylose. The hemicellu- 

 loses must be regarded, therefore, as containing both hexosans 

 and pentosans, but whether in mixture or chemical union is 

 uncertain. While true cellulose constitutes the framework of 

 the plant, the hemicelluloses serve to a greater or less extent as 

 reserve material. In the conventional method of feeding stuffs 

 analysis, the hemicelluloses are found both in the " crude fiber " 

 (109) and in the " nitrogen-free extract " (110). 



19. Lignin. In the young plant, the cell walls consist of 

 nearly pure cellulose. With advancing maturity they become 

 thickened, not only by the formation of additional cellulose and 

 of hemicelluloses but by the deposition of numerous " incrusting 

 substances," the most important group of which has received 

 the collective name of lignin. These substances contain a con- 

 siderably higher percentage of carbon than cellulose (54 to 60 

 per cent) and may be separated from the latter by oxidizing 

 agents. The substances of the lignin group contain methoxyl 

 ( O- CH 3 ) and ethoxyl ( O C 2 H 5 ) groups in considerable 

 amount, and by some are regarded as substituted celluloses. 



20. Crude fiber. The so-called " crude fiber " (109) of 

 plants contains most of the cellulose and lignin of the cell walls 

 and in addition a third group the cutin group 1 whose per- 

 centage of carbon is still higher (60-75 P er cent). Cutin appears 

 to be indigestible. 



21. Starch. Starch is one of the most common and impor- 

 tant of the vegetable carbohydrates. In the growth of plants, 

 starch is formed in the green leaves by the aid of light, and is 

 the first visible product of assimilation. In the mature plant, 

 it is stored up in large quantities in the seed or in the tuber to 

 supply the needs of the new plant. Hence the common grains, 

 corn, wheat, oats, barley, etc., as well as potatoes, are rich in 

 starch and form commercial sources of it. The seeds of most 

 legumes contain it in less amounts but still abundantly. In 



1 Compare Konig: Landw. Vers. Stat., 65 (1907), 55. 



