38o LIFE : OUTLINES OF GENERAL BIOLOGY 



affected by the initial stimulus and its response, before a further 

 reaction can be obtained. 



It is quite conceivable that the word "structure" in the last 

 sentence is to be interpreted in a definite physical sense, as a series 

 of surfaces of the kind we described in the section on permeability, 

 forming some sort of structure within the cell. There is very clear 

 evidence that in many cases, at least, the first response to stimulus 

 is a change in permeability, sometimes of the external film, some- 

 times, perhaps, of internal partitions. The best case is that of the 

 sensitive Mimosa, where the erect position of the leaves and f>etioles 

 depends on the cells being maintained in a state of turgor or dis- 

 tension by water, which in turn is due to the semi-permeable nature 

 of the membrane and the osmotic pressure produced in this way. 

 When the plant is touched, the membranes lose their power to 

 prevent dissolved salts from passing, and the solution escaf)es from 

 the cell; as a result, the tissue loses its rigidity and the cells collapse, 

 so that the leaves move. The same thing is seen in the Venus' fly- 

 trap. In the animal kingdom it is harder to demonstrate this 

 directly, but in the case of the ovum, which responds to the stimulus 

 of the entering spermatozoon (or to certain artificial treatments) 

 by beginning to divide, a considerable increase in permeability is 

 demonstrable. 



The next point to be considered concerns the electric currents 

 produced by the cell, of which we have said little since they were 

 first mentioned in the introductory section on the cell. The first 

 evidence of response in any irritable tissue is an electric one. In the 

 case of a muscle-fibre, there is a sudden electrical change, which 

 may be completed in the "latent period" before the actual con- 

 traction begins. Now it has become plain that there is a very close 

 connection between these electrical changes and alterations in 

 permeability. If there can exist a membrane which is permeable to 

 one of the ions of a salt and not to the other, then there will be 

 a difference of electric potential between the two sides of the mem- 

 brane; any condition which alters the permeability of the membrane 

 with respect to the ions will cause a change in this potential dif- 

 ference, which in turn will produce an electric current. These mem- 

 brane theories of the electric properties of the cell have been worked 

 out for plant cells by Loeb and Beutner; it is shown that mechanical 

 injury of the cell-structure causes an "injury current" due to the 

 local disturbance of the potential difference normally existing on 

 the two sides of the membrane. 



These electrical currents are probably of the greatest importance 

 in the conduction part of the response to stimulus. When the sperma- 

 tozoon enters the egg, a wave of change sweeps all round the surface 

 from the point of entry. Again, if part of a muscle is rendered 

 incapable of contracting by soaking it till it becomes water-logged, 



