28 CELLULAR TISSUE. 



often occurs, which concerns the relative development of mass, on the one hand of 

 the cell walls, on the other of the protoplasmic body and the contents. On the one 

 hand, we have cells with a relatively thin wall, and richly developed protoplasm and 

 contents, characterised by the components of the latter— chlorophyll, starch, sugar, 

 inulin, &c.— as the specific organs of assimilation, and of metabolism, or, chiefly con- 

 taining watery cell sap. On the other hand we find cells whose protoplasm and 

 contents are reduced relatively to the strongly thickened, and often lignified mem- 

 brane, and which accordingly, without giving up the properties of typical cells, or 

 their part in the process of assimilation, obviously participate in the mechanical 

 functions, i. e. the strengthening of the parts to which they belong. The ' CoUen- 

 chyma^ of the cortex of herbaceous plants and the sheaths of the vascular bundles 

 of many monocotyledonous roots are examples of the latter condition. One can 

 accordingly distinguish two extreme forms of structure, and call them shortly ihh- 

 and thick-walled cells, names which are explained by what has gone before. When 

 with the thickening of the wall there appears a process of lignification — which in itself 

 still needs to be more carefully studied — and a hardening of the wall thus occurs, 

 this process will for the future be indicated by the term Sclerosis. 



The different grades of wall-thickening are not generally confined to a definite 

 cell-form, or to any one of the sorts of tissue here distinguished on completely dif- 

 ferent grounds ; there exist isodiametric and fibrous cells with thin, and with sclerotic 

 walls, sclerotic Parenchyma, Epidermis, and Cork cells," &c. But besides this, as 

 may be concluded from what has been already stated, there is no £harp limit between 

 the two main forms, even if one ignores the following fact, which should be brought 

 prominently forward, that sclerosis is the commonest phenomenon of secondary 

 metamorphosis which appears in cells. 



In the large majority of cases, the species and varieties of cellular tissue are dis- 

 tinctly different from one another, and the treatment must start from these cases of 

 marked differentiation and division of labour. But since all are derived from funda- 

 mentally similar meristem, and the properties of the cell remain to all alike, there 

 appear also cases of less complete differentiation and division of labour, and tran- 

 sitional forms, to the existence of which attention must be directed from the very 

 first, and which raise permanent difficulties in many single cases in the way of a sharp 

 division of tissues. 



From the non-equivalent sorts of tissue which originate by metamorphosis of 

 cells, the cellular tissues are, irrespective of their common origin, usually quite clearly 

 distinct. But there are two exceptions to this. Firstly, a sharp limit cannot always 

 be drawn between sclerotic cells and sclerenckyma, which has lost the cell-quality. 

 The secondary sclerenchyma-metamorphosis, which often appears in cells, must 

 lead to transitional forms; and practically it is often impossible to distinguish 

 whether the cell quality remains, or is lost. In many cases therefore the question 

 arises whether a separation of the sclerenchyma from the cellular tissues is to be 

 attempted at all, and to be as far as possible carried out. The frequent occurrence 

 of sharp differentiation answers the question, I think, in the affirmative. 



Secondly, intermediate cases exist between cells and the secretory reservoirs, in 

 so far as the bodies termed secretions, which fill the latter, as oxalate of calcium, 

 resinous bodies, &c., frequently appear also as constituents of the contents of typical 



