142 THE POPULAR SCIENCE MONTHLY 



the semi-permeable membranes of the irritable elements. Perhaps more 

 is known of the relations of membranes to the stimulation-process than 

 to any other cell-activity, and I shall accordingly consider its condi- 

 tions in some detail. 



There is evidence that a rapid and reversible increase in the general 

 permeability of the plasma membrane is an accompaniment and indeed 

 a primary condition of stimulation in irritable tissues. This evidence 

 comes from many sides and is partly direct and partly indirect. Per- 

 haps the clearest indications of this kind are afforded by the motile 

 mechanisms of certain plants, like the sensitive plant {Mimosa pudica) 

 or the Venus's fly-trap (Dioncea) . In Mimosa the characteristic move- 

 ment, which consists of a dropping of the leaves and a folding together 

 of the leaflets, is due to a collapse of certain turgid cells which form the 

 so-called pulvini, or cushion-like masses of parenchyma at the bases of 

 the leaves and leaflets. A fluid containing dissolved substances rapidly 

 leaves these cells on stimulation; evidently the membranes, semi-per- 

 meable during rest, become suddenly permeable to the osmotically ac- 

 tive intracellular substances which maintain the turgor. This expla- 

 nation — first put forward in its essentials by Sachs — is accepted by the 

 majority of plant physiologists, and there is little doubt of its substan- 

 tial correctness. We have here, therefore, an instance where stimula- 

 tion depends directly upon a sudden increase in permeability. Now 

 in this case the primary or critical change is apparently the same as in 

 irritable animal tissues; an electrical variation similar to that shown 

 by an active muscle or nerve accompanies the movement, and the con- 

 ditions which call forth the response are essentially the same in the 

 plant as in the animal. In the case of animals the evidence that in- 

 crease of permeability is a condition of stimulation is, as a rule, less 

 direct. Yet in certain organisms a sudden increase of permeability 

 may readily be shown to be the equivalent of stimulation. My own ob- 

 servations on the pigmented larvae of Arenicola illustrate this very 

 clearly. When these organisms are suddenly brought from sea-water 

 into pure isotonic solutions of sodium salts (e. g., m/2NaCl) the 

 muscles contract with extreme vigor and persistency, causing the larvae 

 (which are small worm-like trochophores about 0.3 millimeter long) to 

 shorten to half their normal length; at the same time the yellow pig- 

 ment contained in the cells of the organism diffuses into the solution 

 and colors the latter yellow. The exit of pigment is the expression of 

 a rapid permeability-increasing or cytolytic action ; this is equivalent to 

 a strong stimulation. If by the addition of any substance to the solu- 

 tion we check or prevent this permeability-increase, we find that stimu- 

 lation is checked or prevented at the same time. Thus, if instead of the 

 pure m/2~Na,Cl we use m/2NaCl to which a little calcium or magnesium 

 chloride, or other appropriate salt, has been added, the strong stimula- 

 tion and loss of pigment are no longer seen — both are simultaneously 



