is also altered, its stem being longer, with considerable intervals between the leaves, which are more diminutive than natural. Placed beside a healthy plant of the same species, the resemblance would hardly be recognised. The diagram represents a species of "linaria," one division of which has been grown in strong, the other in weak light. It would be difficult to adduce a better illustration. Etiolation is familiarly exemplified in the leaves of the cabbage or lettuce, which are more or less blanched, owing to the light having been excluded by compression. Darkness also prevents the plant from fixing the carbon in the form of various secretions: deprives it, in short, of its characteristic qualities. Hence, we bleach celery and endive, in order to correct their acrid flavour.

Light exerts considerable effect in determining the quality of wood in trees. The more carbonic acid these digest under the stimulus of the sun's rays and the more carbon they appropriate, the greater is the density and resisting power of the timber. It is for this reason that trees growing in isolated situations, where they are freely exposed to the sunshine, are preferred to those growing closely packed together in the heart of the forests. The trunks of the latter are generally higher and perhaps straighter; but their wood is inferior in quality. When I see a number of trees or plants growing in a limited space, with access to light on one side only, I am forcibly reminded of a crowd of human beings, each trying to secure the best place for a view of some spectacle. In both cases, the strongest seems to get the best of

* For this I am indebted to N. B. Ward, Esq.

it. Plants always evince a tendency towards the light, which is essential to their well-being, and without which, in the words of an eminent botanist, "they would die of starvation." Poets talk about the feeling possessed by these beautiful members of creation, and dwell upon their instinctive affection for the light. But the fact is simply this. The side of the plant or tree next the light, performs its vital functions far more vigorously than that removed from this stimulus; its foliage becomes more luxuriant, the green colour of its leaves is deeper, and the woody tissue of the stem becoming denser and firmer, draws the weaker and more elongated tissue of the other side with it, and causes the plant to bend towards the light!

Succulent plants with thick leaves, or those which, like the cactus, can scarcely be said to be furnished with leaves, and consequently have but few stomata, demand a greater amount of light, and are generally found growing in tropical countries, in exposed situations. They would seem to require much of this stimulus to promote the degree of evaporation necessary to health! The same may be said of plants with strong fragrance or powerful secretions. Others, whose leaves are thinner and more delicate in structure, do not require so much of this stimulus. Humboldt found flowering plants in the dark mines of Freyburg, and, as Dr. Carpenter observes, mustard and cress have been raised in the dark abysses in the collieries of this country. Many mosses and ferns seem to flourish best in profound shade, on the cool, moist banks of shady lanes, in the clefts of rocks, or the mouths of chasms, where they

illuminate the sombre gloom by the touching beauty of their forms, and the delicate tracery of their sporule-laden fronds!

One of the principal features in the mode of conveying plants from one country to another in the glass-covered cases invented by Mr. Ward, consists in giving the plants a free supply of light, from the exclusion of which they had, prior to this discovery, died by hundreds in their transit.

Many plants close their leaves and flowers at night-time, and appear, as it were, to go to sleep. But this apparent sleep differs from that of animals; for the leaves and flowers, instead of being relaxed like the limbs of animals, are in a state of rigidity which does not usually attend exhaustion. Most of the plants that go to sleep, do so at sunset, their leaves and flowers again expanding at sunrise. The object of this closure may be to protect the delicate parts of plants against the excessive dews of night, or other injurious influences. Decandolle, by exposing a sensitive plant to strong artificial light at night-time, and keeping it in darkness during the day, caused it to change its habits. How many human beings, either from the necessities of their calling, or from habit, have, like the plants under the hands of the French botanist, reversed the appointed order of things, and are wont to recognize the midnight lamp as their noon-day sun!

Light does not exercise so direct an influence upon animals as upon plants. The processes of respiration and digestion in the former are carried on by distinct organs, and not by the surface as in the latter; though light probably

acts as a direct excitant to the nervous system, and therefore affects the supply of nervous influence to the organs by which these functions are performed. The development of the simplest forms of animal life, as of the vegetable germs already noticed, seems to be much determined by this vital agent. "Thus the appearance of animalcules in infusions of decaying organic matter, seems to be much retarded if the vessel be altogether excluded from it. The rapidity with which the small water-fleas, &c., of our pools undergo their transformations, has been found to be much influenced by the amount of light to which they are exposed. And it has been ascertained that if equal numbers of silk-worms' eggs be preserved in a dark room and exposed to common daylight, a much larger proportion of larvæ are hatched from the latter than from the former." The only experiments directly illustrating the influence of light upon the growth and development of animals, are those which were performed by Dr. W. F. Edwards of Paris, and which are extremely satisfactory as far as they go. I notice them with the greater pleasure, as they were not attended with any of the pain which physiologists have occasionally thought it necessary to inflict upon animals, in order to determine some knotty scientific point. happy thought occurred to him, to try the effect of light upon some of those peculiar animals, which, in the course of their existence, undergo distinct change from one form to another; for example, from that of fishes without limbs, and with a tail and gills, to that of reptiles without

The

* Manual of Physiology, by W. B. Carpenter, M.D., F.R.S.

D

the latter appendages and with four limbs. He placed twelve tadpoles in a tin box, which was pierced with small holes for the admission of air, and sunk the box to a depth of several feet in the river Seine. Only two of the twelve underwent the change to reptiles. By subsequent experiments, he showed that the arrest of development was owing to the want of light and not of air; and he found that although the tadpoles did not undergo the usual change to frogs, they continued to live, and increase in size, until they doubled or tripled the usual dimensions. "These experiments," he observes, "unite in proving that the presence of solar light favours development of form. They also show the distinction between this kind of growth, and that which consists in increase of size." Dr. Edwards's experiment actually occurs in Nature,—if without irreverence I may say so-in an animal closely resembling the tadpole in its transition state. is known to naturalists as the Proteus anguiformis, and lives in waters in the subterraneous caverns of Styria and Carniola, where the absence of sufficient light and the low temperature prevent it from assuming the characteristic adult form.

It

That there is some connexion between light and the waking state of animals and man, and between darkness and sleep, is beyond a doubt.

"The starres have us bed;

Night draws the curtain, which the sunne withdraws."

And this harmony of natural phenomena with the life of animated beings is one of the countless wise arrangements of Providence. When the bright sunlight illumines all things, imparting to