CELLULAR STRUCTURE 15 



structural units, in the case of a cell-plate, becomes much more pro- 

 nounced where cells are arranged in three dimensions of space so as to 

 form a cellular mass or cell-body. All the vegetative organs of Higher 

 Plants are constructed on this last-mentioned plan ; they consist of 

 structural elements of the most diverse shapes and sizes, comprising 

 in addition to rounded or polyhedral cells of the isodiametric type, 

 also elements which are tabular or prismatic, attenuated or sharply 

 pointed, stellate or irregularly branched, and so forth. Moreover, not 

 only do the cells of Higher Plants display almost endless diversity as 

 regards their shape, but the character of their w T alls and contents is also 

 extraordinarily diversified. In the case of a Higher Plant more 

 especially among Phanerogams it is not always possible to demon- 

 strate the cellular structure of an organ in its adult condition. Even 

 as late as the nineteenth century it remained a matter of doubt whether 

 the theory of the general occurrence of cellular structure could be 

 extended so as to include water-conducting vessels and latex-tubes. 

 But the researches of Von Mohl and Unger among others have 

 rendered it quite certain that all Higher Plants formerly often 

 designated by I)e Candolle's term of " Vascular Plants," and thus 

 to some extent contrasted with " Cellular Plants " are cellular 

 throughout, at any rate at certain stages of their development. The 

 wood- vessels, though apparently non-cellular in the adult plant, always 

 originate from rows of cells in which the transverse walls degenerate 

 at an early stage ; the same statement applies also to articulated latex- 

 tubes and to certain other histological elements. 



Hitherto in this discussion the cell has been treated merely as the 

 ultimate structural element of the plant-body ; but it must not be for- 

 gotten that every cell represents a unit not only in the morphological 

 but also in the physiological sense. It is in fact a functional as well 

 as a structural element. 2 



If the term " organ " be employed in general to denote the instru- 

 ment wherewith a definite physiological function is performed, then the 

 cell must be regarded as an elementary physiological instrument or " ele- 

 mentary organ." Every cell, namely, performs a definite physiological 

 service either for the whole term of its life or at any rate at some 

 period of its existence, while the sum total of the physiological functions 

 of the various cells constitutes the vital activity of the entire plant. A 

 cell may serve the interests of the organism as a whole, either indirectly, 

 because it possesses a firm membrane or cell-wall, or directly, by virtue 

 of the living cell-body the protoplasmic cell-contents or protoplast 

 enclosed by the cell-wall. This physiological conception of the cell as 

 a functional unit or elementary organ does not imply that every cell is 

 necessarily possessed of independent vital activity. A cell may itself be 



