172 Cellulose 



tives. The subject is of wide interest, involving questions of import, 

 ance to the agriculturist and physiologist ; but the methods by which 

 it requires to be attacked are for the most part purely chemical, 

 and such as are described in more or less detail in this treatise. 



Having now dealt with the more prominent types of ligni- 

 fication in plants and tissues of * annual ' growth, it remains 

 to deal with the lignocelluloses of perennial stems. 



(3) Woods and Woody Tissues. The stem of an 

 exogenous perennial is a complex of structural elements of 

 varied form and function. Of these we may distinguish three 

 main groups : (i) vessels, (2) wood cells proper, and (3) 

 medullary tissue. When compacted together to form the per- 

 manent woody tissue, these groups appear to be indistinguish- 

 able chemically ; they all undergo * lignification ' ; are charac- 

 terised by the same reactions ; and, although it has been 

 stated that they are variously resistant to the action of destruc- 

 tive reagents, the variation has not been satisfactorily referred 

 to any fundamental differences of composition. These points 

 are well discussed by Sachsse in his Chemie u. Physiologic 

 d. Farbstoffe, Kohlenhydrate u. Proteinsubstanzen (Leipzig, 

 1877), and we abstract a short resum'e of his treatment of the 

 subject. 



To the action of the concentrated non-oxidising acids 

 (HC1.H 2 SO 4 ) the vessels are generally more resistant. This is 

 especially the case, however, in the earlier stages of growth. 

 Thus sections of fleshy roots (Daucus Carota\ treated alter- 

 nately with dilute potash and concentrated hydrochloric acid 

 (2-3 days' digestion), are disintegrated by the treatment, the 

 cellular tissue being entirely broken down ; the vessels, how- 

 ever, survive, and are isolated free from the cellular matter in 

 contact with which they were built up. But with older tissues, 

 on the other hand, no such differentiation is observable : the 

 three groups of structural elements are equally attacked by 



