The Poetry of Science; or, Studies of the Physical Phenomena of Nature

by Robert Hunt

These experiments, simple as they are, prove to us the importance of light: the luminous principle of the sunbeam is exciting the vital powers of the plant to decompose carbonic acid and form wood; and the calorific agent, possibly under those modifications which have already been noticed as belonging to the parathermic rays, is essential to the production of flower and fruit.

Observations, which have been extended over many years, prove that with the seasons these solar powers are, relatively to each other, subject to an interesting change. In the spring, the actinic or chemical power prevails, and during this period its agency is required for the vitalization of the germ. As the summer comes on, the actinic rays diminish, and those of light increase. Perhaps it would be more strictly correct to say that the luminous intensity being increased, the chemical power was retarded; the former expression implies a variation in quantity, which may not be correct. We see the necessity for this, since luminous power is required for the secretion of the carbon, with which the woody fibre is formed, and also for the elaboration of the proximate principles of the plant. Autumn, the season of fruit, is characterized by an increase of the heat rays, and a diminution of the others: this change being necessary, as science now teaches us, for the due production of flower and fruit.

The calorific rays of the solar beam, to which the autumnal phenomena of vegetation appear particularly to belong, are of a peculiar character. They have been called, the Parathermic rays, and exhibit a curious compound nature. To these rays we may refer the ripening of fruit and grain, and the browning of the leaf before its fall. May not the rise of the sap in spring be traced to the excitement of either light or actinism, and its recession, in the autumn, to that power from which the plant is found to bend, and which appears to be their modified form of heat?

There can be no doubt that the varieties of climate and the peculiarities of countries, as it regards their animal and vegetable productions, are dependent on the same causes. The distribution of species has been referred by some to specific centres of creation around which the plants and animals have spread, without reference to physical conditions. Although centres of creation may be admitted, these centres themselves have been determined by the physical fitness of each centre to the conditions of the creation, and in like manner the migration of tribes is solely due to these physical forces we have been considering. In every zone we find that vegetable organization is peculiarly fitted for the considerations by which it is surrounded. Under the equator we have the spice-bearing trees, the nutmeg, the clove, the cinnamon, and the pepper-tree; there we have also the odoriferous sandal, the ebony, the banyan, and the teak: we have frankincense, and myrrh, and other incense-bearing plants; the coffee-tree, the tea-plant, and the tamarind.

A little further north we have the apricot, the citron, the peach, and the walnut. In Spain, Sicily, and Italy, we have the orange and lemon-tree blooming rich with perfume, and the pomegranate and the myrtle growing wild upon the rocks. Beyond the Alps the vegetation again changes; instead of the cypress, the chesnut, and the cork-tree, which prevail to the south of them, we have the hardier oak, the beech, and the elm. Still further north, we have the Scotch and spruce fir and larch. On the northern shores of the Baltic, and in that line of latitude, the hazel alone appears; and beyond this the hoary alder, the sycamore, and the mountain ash. Within the Arctic circle we find the mezereum, the water-lilies, and the globe-flowers; and, when the weakness of the solar radiations becomes too great even for these, the reindeer moss still lends an aspect of gladness to the otherwise sterile soil.

The cultivation of vegetables depends on the temperature of the clime. The vine flourishes where the mean annual temperature ranges between 50° and 73°, and it is only cultivated profitably within 30° S. and 50° N. of the equator. To the same limits is confined the cultivation of maize and of olives. Cotton is grown profitably up to latitude 46° in the Old World, but only up to 40° in the New. We have evidence derived from photographic phenomena, that the constitution of the solar rays varies with the latitude. The effects of the sun’s rays in France and England in producing chemical change are infinitely more decided than, with far greater splendour of light, they are found to be in the lands under or near the equator. Indeed, the remarks made on the variations in the character of the sunbeam with the changing seasons, may apply equally to the variations in latitude.

Fungals are among the lowest forms of vegetation, but in these we have peculiarities which appear to link them with the animal kingdom. Marcet found that mushrooms absorbed oxygen, and disengaged carbonic acid. In all probability this is only a chemical phenomenon of a precisely similar character to that which we know takes place with decaying wood. In the conversion of wood into humus, oxygen is absorbed, and combining with the carbon, it is evolved as carbonic acid. Of course we have the peculiar condition of vitality to modify the effect, and we have, too, in this class of plants, the existence of a larger quantity of nitrogen than is found in any other vegetating substance.

These few sketches of remarkable phenomena connected with vegetation are intended to show merely the operations of the physical powers of the universe, so far as we know them, upon these particular forms of organization. During the process of germination, electricity is, according to Pouillet, evolved; and again, in ripening fruits, there appears to be some evidence of electrical currents. Vegetables are, however, in the growing state, such good conductors of electricity, that it is not, according to the laws of this force, possible that they should accumulate it; so that the luminous phenomena stated to have been observed cannot be due to this agency. We know, however, that under every condition of change, whether induced by chemical or calorific action, electricity is set in motion; and we have reasons for believing that the excitation of light will also give rise to electrical circulation.