94 THE CELL 



period, varying in length, of rest or recuperation has elapsed since 

 the cause of irritation was removed. I say advisedly more or less 

 completely, for very often the organic substance is permanently 

 altered in its structure and reacting powers by the application, for 

 a considerable period, of a stimulus, or by the repeated action of 

 the same stimulus. The phenomena thus produced are spoken of 

 as the after-effects of stimulation. 



As a rule, we are not in a position to determine whether or no a 

 body can be stimulated, that is to say, whether it reacts to changes 

 in its environment, since most of the effects due to stimulation are 

 imperceptible to us. Sometimes the protoplasm responds by exhibit- 

 ing movements, or by striking changes of form ; but, as has been 

 just remarked, such phenomena constitute only a small and limited 

 portion of the results produced, although naturally they are the 

 most important to the investigator, since they are apparent to his 

 perception. In consequence, in the following pages, we will chiefly 

 consider the way in which protoplasm responds, by means of move- 

 ments, to the stimuli, which have been grouped into the above five 

 classes. I have therefore decided to commence my considerations 

 of the vital properties of the elementary organism with contrac- 

 tility. 



I. Thermal Stimuli. One of the essential conditions for the 

 vital activity of protoplasm is the temperature of its environment. 

 This temperature can only vary between certain fixed limits; if it 

 oversteps either of these, the protoplasm invariably dies immedi- 

 ately. These limits, it is true, are not the same for all protoplas- 

 mic bodies ; some are able to withstand extremes of temperature 

 better than others. 



The maximum temperature for plants and animals is generally 

 about 40 C. Exposure for a few minutes to such a temperature 

 suffices to cause the protoplasm to swell up and become coagulated, 

 and thereby its irritable structure and its life are destroyed. If an 

 Amoeba is placed in water at 40, it dies immediately ; it draws in 

 its pseudopodia and "converts itself into a globular vesicle, whose 

 sharply defined double contour encloses a large, turbid mass which, 

 by transmitted light, looks brownish in colour" (Kiihne IV. 15). 

 The same temperature causes " death from heat " in JEthalium 

 septicum, coagulation being induced. In Actinophrys, however, 

 instantaneous death occurs at a temperature of 45, whilst the 

 cells of Tradescantia and Vallisneria are only killed by a tem- 

 perature of 47-48 C. (Max Schultze I. 29). 



