THE CELL. 27 



The passive movements which occur in living protoplasm 

 and yet have nothing to do with the life phenomena of the ele- 

 ments moved must not be passed over. Such passive movements 

 characterize the granules in protoplasm, which during rotation 

 and circulation give the appearance of streams of particles mov- 

 ing in various directions. The so-called Brownian molecular 

 movement, which may be observed both in living and in dead 

 cells, also belongs to the passive movements. This consists of 

 an indefinite oscillating (trembling) motion of the granules in 

 the protoplasm which does not change the position of the 

 granules to any extent. The nucleus does not possess the 

 power of independent movement. It is, however, capable of 

 change in shape, as, for example, when a cell is stretched, or 

 forced through a small opening, the nucleus changes in form 

 to correspond with its surroundings. 



2. Irritability is the power of reaction to various stimuli. 

 The stimulus may be mechanical, chemical, thermal, electrical, 

 or due to light. In general it may be said that the stimulus 

 causes an increase or a decrease of the phenomena of life. This 

 depends on the strength and duration of the stimulus. Strong 

 stimulation (e.g., a temperature over 40 C.), causes death to most 

 cells. Cells with active power of motion often move toward, or 

 away from, the source of stimulation. 



If the stimulation takes place by chemical means, we have 

 to do with chemotaxis (chemotropism); and we must distinguish 

 between positive chemotaxis, where the cell moves toward the 

 source of stimulation, and negative chemotaxis, where the motion 

 is in the opposite direction. With certain bacteria and infusoria 

 there are appearances of chemotaxis under the influence of some 

 chemical bodies (e. g., oxygen, citric acid). 



Similarly one may speak of phototaxis (heliotropism), hydro- 

 taxis (hydrotropism), thermotaxis, galvanotaxis, etc. With regard 

 to the last, certain organisms collect about the positive or nega- 

 tive pole on closing or opening a constant electrical current. 



3. The processes of assimilation and excretion belong essen- 

 tially to physiology. Certain details concerning these are spoken 

 of in the discussion of the organs carrying on these functions. 



