THE DIMENSIONS OF CELLS 51 



other words, there is no fixed ratio between the size of an organ 

 and the dimensions of its structural elements ; the cells composing 

 a small organ are fewer in number than those which make up a larger 

 organ of the same kind, not smaller in size. It thus becomes evident 

 why small multicellular plants generally display less histological com- 

 plexity than larger forms, a circumstance upon which Sachs lays 

 considerable stress. For on the one hand complex internal differentia- 

 tion demands the presence of a large number of structural units, 

 while there are on the other hand definite reasons why the individual 

 cells cannot be reduced below a certain size. It should, however, 

 be remarked that, although plants of microscopic size do not as a 

 rule display a great amount of tissue-differentiation, they may never- 

 theless achieve a relatively high degree of internal elaboration in 

 a different manner ; the component cells, namely, may increase their 

 cytological complexity by developing cell organs which are absent 

 in larger multicellular plants. A unicellular Volvocine, for example, 

 is provided with vibrating flagella, with an eye-spot and with a 

 pulsating vacuole, in addition to the usual cytoplasm, nucleus, chroma- 

 tophores and cell-wall. 



Sachs regards the comparative constancy of the size of the cell 

 among Higher Plants very much as the chemist regards the definite 

 value which he ascribes to the atomic weight of an element, namely, 

 as a well-established but at present inexplicable fact. The physio- 

 logical anatomist cannot, however, resign himself to this attitude ; 

 in his continual endeavour to discover the relation between structure 

 and function, he cannot ignore the problem presented by the relative 

 constancy of cell-dimensions. 



It has been stated above that the size of parenchymatous cells 

 varies within comparatively narrow limits, however the plants to 

 which they pertain may differ in size and other particulars, and 

 however diverse the functions of the cells under consideration may 

 be. This circumstance proves that the prevalence of certain average 

 dimensions must involves some physiological advantage, which is not 

 connected with the special performances of the individual elements, 

 but which relates to properties that are common to all living cells. It 

 is impossible at the present time to indicate the precise nature of this 

 advantage. One is forced to assume that the actually observed 

 average cell-dimensions are those which have proved most suitable 

 with reference to rapidity of translocation, control of turgor, adminis- 

 trative activity of the nucleus, in short, all the aspects of metabolism 

 that are in any degree affected by considerations of space. 



In endeavouring to explain the relative sizes of the cells that 

 compose different tissues, one is treading on somewhat firmer ground. 



