16 THE CELL 



quite lifeless, and yet play an important part in the life of the organism 

 as a whole. This statement applies for example to the elements of 

 which cork, fibrous strands and several other important tissues are 

 composed. In such cases indeed it is the cell membrane alone that is 

 cut Misled with a definite task. When once this membrane has been 

 manufactured by the living protoplast, the continued existence of the 

 latter becomes a matter of indifference so far as the welfare of the 

 wlmlc organism is concerned: in such cases, as a matter of fact, the 

 protoplast usually dies, and often disappears altogether, as soon as it 

 is no longer required. The necessity of distinguishing between dead 

 and living cells evidently in no way conflicts with the physiological 

 conception which regards the cell as the functional unit of the plant- 

 body. 



The majority of cells represent not only elementary organs, but 

 also elementary organisms ; in other words, a cell, as a rule, does not 

 merely work in the service of a higher living entity, namely, the entire 

 plaid, but also itself behaves as a living entity, though indeed as one 

 of a lower order of magnitude. Thus each chlorophyll -containing 

 palisade-cell in the leaf of a Phanerogam represents an elementary 

 photosynthetic organ, but at the same time constitutes in itself a 

 complete living organism : if proper precautions are observed, such 

 a cell may be removed from the cell-community to which it belongs 

 without incurring immediate destruction. The author has even suc- 

 ceeded in keeping such isolated elements alive for several weeks in 

 suitable nutrient solutions; the cells continued their photosynthetic 

 activity, and actually underwent an appreciable amount of growth after 

 isolation.' 1 Naturally this power of independent existence is least 

 developed in the case of cells which are highly specialised, and which 

 therefore represent very efficient elementary organs. 



In the preceding discussion it has been shown that cells must be 

 regarded as component parts or elementary organs of the whole plant ; 

 at the same time attention has been drawn to the fact that every cell 

 also constitutes an independent entity possessing a distinct individuality 

 of its own. Clearly every unprejudiced conception of the nature and 

 significance of cellular structure must make equal allowance for these 

 two different aspects of the character of a cell. About the middle of 

 the nineteenth century, when the investigation of the cellular structure 

 of plants was leading to quite unforeseen results, Schleiden, Nageli and 

 other prominent workers were not unnaturally inclined to lay stress 

 upon the individuality of the structural units, and to emphasise the 

 fact of their independent existence as elementary organisms. Schleiden, 

 in particular, adopted a most uncompromising one might justly say an 

 extreme attitude with regard to the matter. According to this view 



