Actual measurements of the osmotic pressure of plant cell have been made. The results are more or less uncertain, because, as has been pointed out, a plant cell is not a definite quantity of uniform protoplasm surrounded by an ideal semi-permeable membrane, but is instead a mass of living matter which is approximately semi-permeable throughout its entire volume and is in a constantly changing condition because of the anabolic and catabolic activities which are going on in it; but values have been obtained which show a normal osmotic pressure as high as fourteen atmospheres in the cells of very turgid plants, such as those of some of the green algæ. Animal cells probably have an osmotic pressure similar to that of the blood which circulates around them, which is approximate that of seven atmospheres.
SURFACE BOUNDARY PHENOMENA
In the preceding chapter, a brief consideration of the phenomena arising at surface boundaries was presented. It was pointed out that when any substance exists in the colloidal, or dispersed, condition, it has relatively enormous surface area and that, consequently, enormous surface boundaries between the dispersed phase and the dispersion medium exist in all colloidal mixtures. Since protoplasm is conceived to exist in the form of a colloidal gel, having a foamlike structure, it is apparent that it has these enormous surface boundaries between the different phases of the system, and that the phenomena arising from this condition are of great importance in its biological activities. The following necessarily brief discussion will serve to give some indication of the physiological importance of the surface boundaries in such a system.
It is easy to see that the molecules which are in the surface layers at the interface, where two phases of a colloidal system are in contact, are under the influence of forces quite different from those which are acting upon the molecules in the interior of either phase. It is apparent that the molecules in the surface layer are exposed on the inner side to the attraction and influence of similar molecules, while on the opposite, or outer, side they are exposed to the influence of molecules of an entirely different kind. This results in a state of tension, known as "surface tension," with the development of resultant forces and energy which profoundly affect the chemical reactivity of the molecules which are present in this surface layer. The so-called "surface energy," which results from this surface tension, produces marked increases in the possibility of chemical reaction between the materials which are present at the surface boundaries. In colloidal gels, this effect is so pronounced, in many cases, as to completely overshadow other types of influences upon reaction velocities. Also, the surface layer of a liquid is compressed by its surface tension, to such an extent that the solubility of substances in this surface layer is greatly increased over that of the same substances in the interior of the liquid, which results in greatly increased concentration of dissolved substances in the surface layer, and so increases the rate of chemical changes which take place there, as contrasted with the rate of the same reactions going on in the interior of the solution. This latter consideration seems to be the factor of largest influence in colloidal catalysis.
But in addition to the increased rate of reaction in the surface layer due to the increased energy available there and to the increased concentration of dissolved substances, there is the possibility that the act of concentration itself bring into play molecular forces which give rise to a resultant increase in chemical potential, or chemical affinity, of the reacting materials, such as has been observed to result in other concentrated solutions. A discussion of the theoretical and mathematical considerations upon which this conception is based would be out of place here, but there is ample experimental evidence to indicate its soundness.
Further, as has been pointed out, colloidal phenomena are essentially due, in large part at least, to the electric charges on the dispersed particles. Electric charges accumulate at the surface of any charged body. Hence, the surface layers in any colloidal system carry its electric charges in highest concentration, and all of the chemical changes which are stimulated by electrical phenomena are most strongly influenced at the surface boundaries between the different phases of the system. This latter consideration affords a satisfactory explanation of the well-known depressing, or stimulating, action of electrolytes, especially acids and bases, upon the enzymic catalysis of protoplasmic reactions.
These few, brief statements are sufficient to indicate how extensively the chemical activities of colloidal protoplasm are influenced by the phenomena arising from the surface boundaries between different materials, which are present in such enormous extent in a colloidal gel. Surface boundary phenomena in a heterogeneous system, such as we have seen protoplasm to be, provide the possibilities for many reactions which would otherwise take place very slowly, if at all. Mere subdivision of the protoplasmic materials into the film, or foam, structure brings into play energies which may predominate over all other types of energy in the system. Here, too, effects may be extraordinarily modified by slight changes in environment, which effects could not be explained by any considerations which govern ordinary chemical reactions. Here, we deal with adsorption and other colloidal phenomena, rather than with ordinary stoichiometric combinations.
Indeed, it is not too much to say that the differences between the chemical phenomena which are called "vital" and those which take place in ordinary laboratory reactions are due to the fact that the former are manifestations of the interchanges of energy between the different phases of a heterogeneous colloidal system, while the latter are governed by the laws of ordinary stoichiometric combinations.
ELECTRICAL PHENOMENA OF PROTOPLASM
The investigations of this phase of the physical chemistry of protoplasm have dealt almost exclusively with animal tissues and reactions, and have included the study of such phenomena as nerve impulses, muscular contractions, heart-beats, glandular secretions, etc. Tissues which respond to nerve, or brain, control are, of course, not found in plants. But there is plenty of experimental evidence to show that plant protoplasm carries electrical charges and exhibits electrical phenomena which are similar in character to those of animal tissues. In fact, it has been shown that the contraction of the lobes of the Venus' fly trap, when they close over an imprisoned insect, are accompanied by electrical phenomena in the leaf tissues which are precisely similar to those which take place in an animal muscle when it contracts. It seems probable that many of the observations and conclusions which have been derived from the study of the electrical disturbances in animal tissues may later be found to have definite applications to the vital phenomena of plant cells. Hence, it seems proper to give some brief consideration to these matters here.