THE VITAL PROPERTIES OF THE CELL 135 



rated from the living body. Hence, in examining a tissue outside 

 of the body, blood serum, aqueous humour, amniotic fluid, iodised 

 serum, or artificially prepared mixtures resembling these fluids, only 

 function, to a certain extent, as indifferent, supplementary fluids. 

 As a matter of course, they cannot at all supply the natural condi- 

 tions for the cell. 



In endeavouring to understand the relationship which exists be- 

 tween the cell and the fluid which bathes it, care must be taken at 

 the outset to avoid the idea that the former is simply saturated by 

 the latter. Such a conception is wholly fallacious ; on the con- 

 trary, each cell is an independent unity which selects certain sub- 

 stances from the mixture of fluids surrounding it, and absorbs a 

 varying quantity of them, whilst others it quite rejects. In all 

 these respects different cells behave very differently : in a word, 

 the cells, to a certain extent, make a selection from the substances 

 offered them. 



Such selective powers, often very different in character, may be 

 easily demonstrated by the following : 



Amongst the lowest unicellular organisms there are some which 

 possess silicious skeletons, whilst others construct theirs out of 

 carbonate of lime. Hence they exhibit quite opposite powers of 

 selection towards these two substances, both of which occur in 

 small quantities in solution in water, and by this means very im- 

 portant effects have been produced in the formation of chalk, and 

 of the geological strata, consisting of silicious shells. Similarly, 

 different plants, which thrive side by side under similar conditions 

 and in the same water, take up from it very different salts, and 

 these in very varying quantities. The relative proportions which 

 occur may be easily computed by drying and burning the plants, 

 and then reckoning out the proportion which the ash bears to the 

 whole of the dried substance, and further the proportion the 

 separate constituents of the ash bear to the pure ash. 



The ashes of several kinds of Fucus which were collected on the 

 west coast of Scotland were examined, and the results obtained 

 were tabulated by Pfeffer (V. 23) in his Plant Physiology. 



