452 PRINCIPLES OF SOIL MICROBIOLOGY 



Chemistry of hemicelluloses. The hemicelluloses are amorphous 

 polysaccharides, but are distinguished from the celluloses by their 

 easy solubility in dilute alkalies and in hot dilute acids, such as 1 per 

 cent HC1. Some give the brown to black iodine reaction like true 

 celluloses. On hydrolysis hemicelluloses give glucose, mannose, galac- 

 tose, or mixtures of these, xylose and arabinose; the hemicelluloses 

 are thus termed dextrans, mannans, galactans, mannogalactans, 

 pentosans (xylans, arabinans), according to the constituent mono- 

 saccharides. Hemicelluloses are present not only in higher plants, but 

 also in algae, fungi and lichens (lichenin). Hemicelluloses of plant 

 seeds are insoluble in water, diastase solution (differentiating them 

 from starches), cold dilute KOH, but are more readily hydrolized 

 by acids into sugars than the celluloses. In relation to their role in 

 the plant tissues, celluloses are commonly believed to serve as pro- 

 tective substances in plants and are unaffected by plant metabolism, 

 while hemicelluloses are reserve materials in the plants, which must 

 be brought into a soluble form by means of enzymes, before they 

 can be utilized in plant nutrition; in some plants they serve for 

 structural purposes holding the fibers together. The hemicelluloses 

 thus comprise two different groups of substances: (1) the reserve 

 celluloses (mostly mannans) of seed, and to some extent of grasses; 

 (2) supporting substances, mostly galactans and pentosans, having a 

 mechanical function. The reserve hemicelluloses and starches can take 

 the place of one another, so that seeds poor in starch are rich in 

 reserve celluloses and vice versa. 



The pentosans are present in the cell walls of all green plants, in the 

 bark and woody fiber of trees, in mosses, fungi, seeds and fruits. They 

 occur very abundantly in straw which usually contains 23 to 29 per 

 cent pentosan. Corn cobs contain 32 per cent pentosan; pine needles 

 6.8 per cent, oak leaves 10.3 per cent. Older tissues contain larger 

 quantities of pentosans than younger ones. According to Van 

 Hulst, Peterson and Fred, 75 the pentosan content of the corn plant 

 increases from 7.4 per cent in the kernel to 31.8 per cent in the cob at 

 maturity. 76 The pentosans are probably not reserve materials but 



75 Van Hulst, J. H., Peterson, W. H., and Fred, E. B. Distribution of pento- 

 sans in the corn plant at various stages of growth. Jour. Agr. Res., 23: 655- 

 663. 1923. 



76 The method of determination of pentosans is given by Pervier, N. C, and 

 Gortner, R. A. The. estimation of pentoses and pentosans. Jour. Ind. Engin. 

 Chem., 15: 1167, 1255. 1923. 



