movements of the fugitive hearts of the laticiferous web of vessels.

These remarks upon the phenomena of cell growth, are intended to serve rather as suggestions than as explanations; but while we would be careful not to assign to them an importance they do not deserve, we must be equally on our guard not to scout the idea that physical agents are concerned in these changes. Heat is undoubtedly concerned, and largely so, and it is to be remembered that the changes in the organic substance accord with the natural law of action of this agent in inorganic substances.

The phenomena of cell-growth point also to the influence of light as well as heat. This agency is necessary to the free formation of the granules of starch: thus potatoes grown in orchards thickly crowded with trees, are always more watery and less rich in amylaceous matter than those cultivated in the open fields. The growth of the layers which impart hardness to wood requires a free exposure to the influence of light; and for this reason the timber grown in dense forests is never so firm and hard as that which has been freely exposed to the sun. The granules of the substance allied to starch, which impart colour to plants, depend for their formation upon the same foreign power. The growth of the entire plant is also influenced by light, and thus the trunk and its branches

are always more perfectly developed than the root and rootlets.

In all these changes light seems to determine the formation of additional nuclei or centres of growth, and to be the prime mover of the act of nutrition. As heat stands related to development, so light appears to be connected with genesis: how and by what means is not easy to decide, but still we may obtain much information if we study the fact attentively. Light, it may be said, involves a certain clearness of the atmosphere and absence from clouds, which is favourable to evaporation and radiation; and under these circumstances it may be supposed that the organic fluid in which the nuclei originate tends rapidly to give off its more watery portions. This process proceeds until at last the remaining fluid is no longer able to retain the organic matter in solution, and hence perhaps the appearance of the solid particles. In this way the formation of nuclei in the organic fluid, and of crystals in an ordinary chemical solution, are both favoured by the operation of light, for a state of atmosphere is involved in the presence of this agent which would bring about the necessary evaporation of the solvent.

I am not prepared to deny that light has a special action upon growth, for it is directly concerned in the changes of the photographic process; but at the same time it is to be borne in mind that this agent is cor

relative with physical motion, heat, chemical affinity, and electricity. Instead of having any distinct and independent mode of action, therefore, light is rather to be regarded as a condition of one and the same action, higher in character, perhaps, than heat, but still related to this and the other physical agents, just as the several prismatic colours in light are related to each other, all being varying aspects of one central principle.

It is difficult indeed to imagine any real difference in the mode of operation of the several physical agents foreign to the body; and it is more consistent with the spirit of philosophy to seek in secondary causes for the explanation of any apparent differences. Instead of supposing light to have any special action upon the organism, it is better, therefore, to regard this influence as expressive of that condition of the atmosphere most favourable to evaporation and radiation, and most likely to bring about that degree of inspissation in the organic fluids which naturally results in the deposition of solids that were previously in a state of fluidity.

The phenomena of "rotation" refer also to the agencies concerned in the free discharge of the other functions of nutrition; and here, likewise, it is possible to meet with a partial explanation without having recourse to the unintelligible mystery of vital power. Let it be supposed indeed that the cell (itself acted

1

upon by external agents) reacts inductively upon the floating nuclei contained within it, and it may be imagined that one of the consequences of the action will be the communication of a motion to these nuclei, such as is witnessed in a constrained and modified manner in the various forms of electromotory machines. These intercellular movements may be in part explained in this way, for there is no doubt of the existence of a tangential force in connexion with electricity, which under some circumstances is sufficient to communicate to the body acted upon a direct, and, as it were, orbital movement around the body which is the principal source and spring of action.

At the present stage of our inquiry we may only hope to obtain presumptive arguments, for those which furnish demonstration and conviction have to be reared by a tedious cumulative process, which will not be complete until all the phenomena of vital movement have passed under consideration. At the same time it is necessary to place a check upon ourselves, lest we be blinded by our prejudices, and to keep the judgment cool and collected while we weigh the evidence which will gradually come under notice.

2. Of intra-organic force as the agent in cellular

movements.

As in the phenomena of cyclosis, it is probable that in this case there are internal physical agencies which

In the

originate and operate in the same manner. plant, however, the power which enkindles and fosters life proceeds in great part from without; and it is not until we ascend to animal bodies that the inherent life of the creature assumes any great degree of prominence. In the laticiferous movement, which approaches most nearly to the capillary circulation of animals, there is, it is true, the distinct trace of internal power; and so, also, it may be argued, in the molecular movements of "rotation," and in cells which exhibit laminated deposits; but in all these cases the organic and foreign forces operate in the same manner, and are analogous in character; and it is not necessary to attempt to determine what precise degree of importance belongs to either.

The evidences, moreover, of the operation of any vital principle in the vegetable economy are too obscure to allow satisfactory and advantageous investigation, and it is better, therefore, to leave the contested topic in complete abeyance until we arrive at the more marked problems of animal life.

III. OF THE GENERAL MOVEMENTS OF THE SAP.

This question is much more simple than it appears to be and if we take care to suspend the operation of certain unfounded prejudices which appertain to it, we shall find that the general movements of the sap