THE FUNCTIONS OF PROTOPLASM AND CELLS 



53 



vertebrate animals is decomposed by light. The electric organs of 

 certain fishes produce a series of discharges. 



The nature of the response is determined by the nature of the respond- 

 ing protoplasm, not by the kind of stimulus. A muscle cell contracts, 

 whether the stimulus be chemical or electrical. A gland cell secretes, 

 and its product is always the same, regardless of what started its 

 activity. 



The extent of a response is, in general, rather definitely fixed for any 

 given cell. If the cell responds at all, it does so to its full capacity. An 

 organ made of many cells may respond in various 

 degrees, depending on whether few or many of its 

 component cells join in the response. How many 

 cells respond depends on the intensity of the stimulus. 

 Each individual cell, however, follows the all-or-none 

 rule of acting either at its maximum capacity or not 

 at all. 



What Is Living Matter? — The characteristics of 

 living matter enumerated in the opening paragraph of 

 this chapter do not constitute a criterion which would 

 enable even an expert to say in a specific instance 

 whether a bit of matter were alive. Application of 

 the rules would occasionally be futile. The chemical 

 composition of recently killed protoplasm would, on 

 analysis, be indistinguishable from that of living proto- 

 plasm; but something intangible would be gone from 

 it. Spontaneous movement and change of shape may 

 occur in a drop of liquid, under certain circumstances, 

 because of changes in the surface film. Moreover, 

 living things in the form of resting spores exhibit no 

 detectable movements over long periods of time. A crystal may be made 

 to convert part or all of itself into a flock of smaller crystals, in a way 

 that would be hard to exclude in a definition of reproduction. Finally, 

 metals respond to things in the environment, such as a magnet or 

 electric potential. 



A definition of life which lists the ordinary activities or conditions of 

 Uving things is feasible; but it could not be used practically for a 

 complete classification of all objects into two categories, living and 

 nonliving. 



A B 



Fig. 34.— Fla- 

 gellura ifl) of Eu- 

 glena, showing 

 (right) contractile 

 threads within it. 

 (B after Dellinger 

 in Journal of 

 Morphology.) 



References 



Heilbrunn, L. V. An Outline of General Physiology. W. B. Saunders Company. 

 Marsland, D. Principles of Modern Biology. Henry Holt and Company. 



