236 Cellulose 



as the seat of the essential vital operations of elaboration and 

 metabolism, the fibrous and vascular systems in addition to 

 taking their specialised part in the general distribution of 

 nutritive and other matters are the strengthening elements, 

 whereas the cuticular tissues are mainly concerned in closing 

 off and protecting the tissues beneath from the unregulated 

 action of water and air. In regard to the chemical relationship 

 of the tissue-substances to the vital processes by which they are 

 formed, and to which they in turn contribute, we have little 

 but indirect evidence and conjecture to go upon. So far as 

 this evidence is of a purely chemical nature, it has been dealt 

 with in the preceding sections. But it depends in great 

 measure upon the results of investigation by physiological 

 methods, and for such results the special treatises must be con- 

 sulted. We have to deal, in conclusion, with those changes of 

 the tissue-substances, celluloses and compound celluloses 

 which accompany or follow the cessation of vital activity. 



The term ' death ' is perhaps more difficult to apply to the 

 vegetable than to the animal organism. In a perennial plant 

 the active life, in the sense of the elaboration of new material, 

 is bound up with portions only of the structure. In an 

 ordinary forest tree, for instance, the leaves are these active 

 agents ; the trunk or stem tissues, on the other hand, are largely 

 depleted of the organic nitrogenous matter (protoplasm) upon 

 which the vitality depends ; they have ceased to live in the full 

 sense of the term, but, on the other hand, they are known by 

 casual observation to live, in the sense opposed to decay. 

 There are, therefore, various phases of life in the plant recog- 

 nised by ordinary observation, and more exactly defined by 

 the physiologist ; but these phases graduate by insensible stages 

 into the region where decay and chemical disintegration are 

 predominant. If, therefore, it is difficult for the physiologist 

 to draw the line between the life and death of a cell, it is still 



