IRRITABILITY 229 



fore more sensitive to external influences and is more likely 

 to change than is a concentrated solution. Some of the 

 substances in the cell-sap may be present in such minute 

 quantities as to be dissociated. In this case these dissoci- 

 ated atoms, and in consequence the cell-sap as a whole, 

 would be very sensitive to external influences, physical and 

 chemical. Anything influencing the cell-sap in any way 

 immediately affects the protoplasm through the cell-sap 

 which permeates every part of it. This influence upon the 

 protoplasm may be chemical or it may be electrical. Only 

 the future can decide which it is, but meantime this concep- 

 tion assists us in appreciating how the protoplasm may be 

 affected and how the stimulus may be promptly transmitted 

 from cell to cell.* 



Any substance or any force which disturbs the balance 

 between the constructive and destructive processes main- 

 tained by the organism or the cell will affect the organism, 

 producing an irritation equal to the disturbance. If the new 

 balance necessitated by the introduction of the new sub- 

 stance or the new force is more or less favorable to the 

 accomplishment of the vital processes than the old, the liv- 

 ing cell or organism will respond accordingly. Organisms 

 living in certain situations are exposed to very frequent and 

 very rapid changes in their environment. Thus the plants 

 living between the high and low tide marks on the sea- 

 coast are subjected to a twice daily change in their rela- 

 tions to water. If rain falls upon them while they are 

 bared by the tide, they are subjected to still another change. 

 Marine plants living at the mouths of fresh-water streams 

 are also exposed to frequently repeated and very rapid 

 changes in the composition and density of the medium which 

 surrounds them. Such changes in osmotic influences imply 

 a very great range of adaptability on the part of the 

 plants regularly exposed to them. Plants living in situa- 

 tions where osmotic influences change only gradually or 

 slightly, as in pools which dry up during the summer, or in 

 bodies of water which are added to only occasionally or 



* Matthews, A. P. The nature of the nerve impulse. Century Magazine, 

 vol. 63, 1902. Abstract also in Science, vol. 15, p. 344, 1902. 



