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 conditions 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. Perhaps 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 (Dionæa). In Mimosa the characteristic movement, 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-permeable during rest, become suddenly permeable to the osmotically active intracellular substances which maintain the turgor. This explanation—first put forward in its essentials by Sachs—is accepted by the majority of plant physiologists, and there is little doubt of its substantial correctness. We have here, therefore, an instance where stimulation 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 conditions which call forth the response are essentially the same in the plant as in the animal. In the case of animals the evidence that increase 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 observations on the pigmented larvæ of Arenicola illustrate this very clearly. When these organisms are suddenly brought from sea-water into pure isotonic solutions of sodium salts (e. g., ) 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 pigment 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 solution we check or prevent this permeability-increase, we find that stimulation is checked or prevented at the same time. Thus, if instead of the pure we use to which a little calcium or magnesium chloride, or other appropriate salt, has been added, the strong stimulation and loss of pigment are no longer seen—both are simultaneously
