230 INJURY, RECOVERY, AND DEATH 



discussion of the nature of the cell surface.^* Enough has 

 been said to show that there is considerable evidence that 

 there is a layer at the surface which is different from the 

 underlying protoplasm and that some substances pene- 

 trate it more rapidly than others. It is doubtful, whether 

 there are many substances to which it can be regarded as 

 wholly impermeable. It is, however, able to protect the 

 metabolism of the cell from various kinds of interference 

 from without, and to provide for the differentiation of 

 multicellular organisms by making it possible to keep 

 various processes separate. The principal advantage 

 of cell division may consist in providing the semiper- 

 meable membranes, which make differentiation possible. 



A good illustration of this differentiation is seen in 

 those cases where diverse chemical operations go on in 

 adjoining cells without mutual interference. In many 

 plants deeply colored cells are surrounded by colorless 

 ones, and the soluble coloring matter does not show any 

 tendency to diffuse into the surrounding cells. We may 

 even observe that the color is confined to the vacuole of 

 the cell, and does not diffuse into the surrounding proto- 

 plasm. In the same way we observe in some plant cells 

 colored plastids (chromatophores) containing soluble 

 pigments which do not diffuse out into the cytoplasm. A 

 cell of this sort is shown in Fig. 96. 



In the case of Griffithsia, each of these plastids is 

 surrounded by a semipermeable membrane which retains 



°' Czapek (1914) has suggested that the plasma membrane is com- 

 posed of soaps. Nathanson (1914) regards it as mosaic of lipoid and 

 non-lipoid particles. This would not provide an entrance for lipoid- 

 soluble and lipoid-insoluble substances into the cell-sap unless each ele- 

 ment of the mosaic extended continuously, without a break, from the 

 outer surface to the vacuole. For a general summary see Bayliss (191.'5), 

 Hober (1914), and McClendon (1917). 



