238 Cellulose 



of cellulosic tissues is a frequent phenomenon ; the process 

 by which the tissue is broken down is of the character of an 

 ordinary hydrolysis, i.e. is determined by enzymes, and postu- 

 lates therefore a cellulose susceptible of molecular disaggrega- 

 tion. The ' hemi-celluloses ' (p. 87) are compounds of this 

 order, and occur chiefly in seed-tissues, where they serve as 

 reserve materials for the early growth of the embryo on germi- 

 nation. The more resistant celluloses and compound cellu- 

 loses are obviously destined for more permanent functions, and 

 they have been dealt with in this treatise under the term * per- 

 manent tissue.' But permanence is a relative term. We have 

 already endeavoured to trace the changes and modifications 

 which these substances undergo in the normal life of the 

 plant : lignification, for instance, has been dealt with as a 

 progressive chemical transformation of celluloses, differing 

 probably ab initio from those of the normal type ; and evidence 

 has been given in terms of the constitution of these compounds, 

 showing them to be highly reactive and susceptible of modifi- 

 cation in various directions under treatment in the labora- 

 tory. 



We have now to follow the fate of these substances in the 

 ordinary processes of the natural world. Under these cir- 

 cumstances, * death ' is succeeded by a variety of processes of 

 decay and destruction. These are in part intrinsic, in part 

 determined by outside agencies ; they are of the two kinds : 

 processes of resolution, and processes of combination or con- 

 densation, which are usually concurrent. The former have 

 been dealt with on p. 66. These are fermentation processes 

 attended with evolution of gaseous products. The latter are 

 defined by an extended series of their products, viz. humus, 

 peat, lignite, and the coals. The chief characteristic of this 

 series of degradation products of plant tissues is the accumula- 

 tion of carbon at the expense of oxygen and hydrogen. This 



