170 



BURTON EDWARD LIVINGSTON 



leaves the wall, and we have virtually a naked cell, for the wall 

 now takes no part in determining the pressure equihbrium. 

 Such a cell is of course plasmoh^zed. 



It is clear that, when the cell environment consists of other 

 cells, no forces of different nature from those already considered 

 need be taken into account. If a tissue be in osmotic equilibrium, 

 all of the various forces already- dealt with must just balance one 

 another; otherwise one cell must tend to enlarge and another to 

 contract until such equilibrium be attained. 



I have reviewed these fundamental features of cell physics in 

 order that we may have clearly in mind the somewhat complex 

 interaction of forces which must be supposed to determine the 

 degree of turgidity present in a cell and, in the absence of equilib- 

 rium, the direction of water movement through its wall. Most 

 writers in this field have neglected the component forces and their 

 complicated inter-relations, gi\'ing attention usually only to 

 the apparently simple balance between vacuolar and en\'iron- 

 mental osmotic pressure. Plant physics has now proceeded far 

 enough, however, so that it frequently becomes requisite to con- 

 sider other forces than these two; indeed, no very clear concep- 

 tion of the relations obtaining between a cell and its surroundings 

 is possible in any case unless the complex interplay of forces which 

 has been here outlined is taken into account. The details of cell- 

 di\dsion, of wall formation and of cell growth, as well as those of 

 water loss and water intake, and probably also those of manj^ 

 so-called regulatory responses, seem likely to be comprehended 

 only through as thorough an analysis as is possible of the physical 

 conditions which hold in the cell and in its environment. 



I wish now to turn attention to certain aspects of the environ- 

 mental complex and to make tentative inquiry regarding causal 

 relations which may be discoverable between certain environ- 

 mental conditions and the forces which we have been considering. 

 Perhaps the simplest mode of approach to the questions which I 

 have in mind is to suppose changes to take place in the surround- 

 ings and try to picture the main results of such changes as they 

 become effective in the cell. 



Let us begin wdth a walled, vacuolated cell, turgid, and sur- 



