1 86 Cellulose 



have also investigated this question by studying the germina- 

 tion of barley. In the germination of barley there is not only 

 an increase in the ' total pentosan,' but the early permanent 

 tissue is found to contain a considerable proportion of these 

 furfural-yielding constituents. Of these constituents, moreover, 

 more than 80 p.ct. resist the process of alternate digestion in 

 cold dilute acids and alkalis ; they are not therefore pentosans 

 in the ordinary acceptation of the term. On the other hand, 

 the pentosans proper are found in relatively large proportion 

 in the later stages of growth of the cereal straws ; and, again, 

 the evidence leads us to regard the pentosans as secondary 

 products of metabolism, in contradistinction to primary products 

 of assimilation. It is evident from this brief outline that the 

 physiology of the pentosans their origin, fate, and general 

 significance is still, in many directions, problematical. In 

 regard, however, to the narrower problem of lignification, we 

 may sum up the evidence as follows : The formation of furfural- 

 yielding products invariably accompanies lignification. These 

 products exist in the earlier stages of lignification in the 

 cellulosic form, but with age (perennial stems) are gradually 

 transformed into pentosans of relatively low molecular weight, 

 and ceasing to occupy any organic relationship to the tissue. 

 The proportion of these constituents is uniform (18-24 p.ct.) 

 over a wide range of woods hitherto investigated, and varies, 

 moreover, but little with the age of the wood ; the proportion 

 is, however, much less in the woods of the Coniferae (6-9 p.ct.), 

 which therefore represent lignification of another chemical 

 type (see p. 197). So far no relation has been traced between 

 the percentage of ' pentosan ' and the physical properties of the 

 woods. 



Before passing from this section of the subject we must 

 describe somewhat more in detail the characteristic product, 

 wood gum, already briefly noticed. 



