OSMOTIC PRESSURE AS A FACTOR 



173 



ess of incipient drying be carried far enough, it of course results, 

 consecutively, in loss of turgidity, wilting, death from water loss, 

 and finally desiccation. 



But in our supposed experiment, the evaporation of water from 

 the cell wall might proceed at such a slow rate that sufficient 

 time might elapse for various changes to occur in the nature of 

 the wall — as its impregnation with waxy materials, for example — 

 and if such reactions occurred the vapor tension of the wall and its 

 water loss by evaporation might be enormously decreased. Such 

 reactions occur commonly in leaves raised out of water by growth 

 and in the emergence of aerial leaves from the practically satur- 

 ated and enclosed atmosphere of the bud. A parallel phenomenon 

 no doubt occurs in animals when they pass from an aqueous to 

 an aerial environment, as frequently at the time of birth, ekdysis, 

 etc. Reactions to incipient drying must be common in the super- 

 ficial cells of subterranean plant roots, when subjected to pro- 

 gressive dessiccation of the soil. It also, appears that the produc- 

 tion of sporangia, conidia, etc., by the common moulds may be 

 related to the altered relations between the force of wall imbibition 

 and the osmotic force within the cell, initiated by the action of 

 increased water loss from exposed wall surfaces. 



It is apparent that the general principles just outlined are 

 applicable in those cases where no vacuole exists and where it 

 may be possible that the outwardly directed force of the cell is 

 finally imbibitional in the protoplasm rather than osmotic in the 

 vacuole. It is also apparent that the water-extracting power of 

 the environment ma}^ be due to the force of crystallization, as 

 when the surrounding water freezes, as well as to osmotic, imbibi- 

 tional, capillary and evaporational forces. I shall refrain, how- 

 ever, from any specific discussion of these various cases. 



Most cells that are exposed to the air, and consequently to 

 evaporational water loss, are not so exposed on all sides. Gener- 

 ally they are peripheral cells of a tissue mass and are directly in 

 contact with other, deeper-lying cells, from which water may be 

 withdrawn. The deeper cells are usually in contact, intermit- 

 tently at least, with an environmental solution, and diffusion from 

 behind — as we may say — tends to prevent desiccation through 



