The leaves of the sida or althea Theophrasti, of the ayenia, and cenothera, are placed alternately. Though horizontal, or even depending during the day, at the approach of night they rise, embrace the stem, and protect the tender flowers.

"The leaves of the solanum or night shade, are horizontal during the day; but, in the night they rise and cover the flowers, The Egyptian vetch erects its leaves during the night, in such a manner that each pair seem to be one leaf only. The leaves of the white lupine, in the state of sleep, hang down and protect the young buds from being injured by the noc

turnal air."

The flowers also, as well as the leaves, have the power of moving. During the night, many of them are inclosed in their calyx. Some, as of the German spurge, geranium striatum, and common whitlow grass, when asleep, hang their mouths toward the earth, to prevent the noxious effects of rain or dew, It is probable that such flowers are not defended by their leaves.

It would appear that this sleep of plants was designed for the perfection of the seed. For, those plants, the seed-receptacles of which are sufficiently secure, never sleep; and a plant after fructification sleeps no more.

The cause of these movements in plants has been ascribed to the presence or absence of the sun's rays. Some motions are evidently excited by heat. But plants kept in an equal temperature in a hot-house,

fail not to contract their leaves, or to sleep, in the same manner as when they are exposed to the open air. This fact evinces, that the sleep of plants is rather owing to a peculiar law, than to a quicker or slower motion of the juices.

All the facts I have enumerated on vegetable life, tend obviously to prove that plants are endowed with internal powers of self preservation quite independent of man, which watch over them continually, in a manner as incomprehensible, yet as efficient, as the instinctive actions of animals. And this power, for want of a better name, or a better knowledge of the cause, has been termed the principle of vegetation. But, in what manner the motions of plants just noticed differ from the motions which are thought to depend on a distinct principle belonging to a muscular fibre in animals, we are quite ignorant. It is hardly likely that the fibrous texture of plants and of their vessels, though possessed of elasticity, has any thing like the property of alternate contraction and relaxation known to exist in the true muscular fibre of animals; yet plants, as we have seen, are excited to very curious movements. So that in the infinitely wise economy of nature, similar effects may be accomplished by a different organization. Varied, however, as this may be, there is still something beyond which is inexplicable. We cannot doubt that the structure of leaves and flowers is as well adapted to the delicate stimuli acting upon them to excite their movements, such as light and heat, as the structure of animals is

to the grosser kind of elements with which they come in contact. On whatever principle vegetable motions depend, elasticity or irritability, it is generally allowed that the motions of animals arise from the irritability of a muscular fibre. This irritability, however, though admitted as a fact, is obscure or inexplicable in its cause and mode of operation. It is no less mysterious how a simple muscular fibre should possess the inherent power of contraction and relaxation, should become weak or strong by exhaustion or tone; than it is how the simplest animal structure should be able to commence, continue, and suspend its spontaneous motions.

SECT. II.

Of Animal Motions.

Beyond the sphere of vegetation, the system of vital irritability is presumed to commence; but the barrier is so ill defined, that we can scarcely tell where the first actually terminates, and where the latter begins.

Notwithstanding the subject of muscular motion has been investigated by some of the most learned and ingenious men, it still remains involved in the greatest obscurity. A medical writer well observes, that "although many curious observations have been made, and as far as the laws of dead mechanism can be ap

plied to a living machine, the investigators may have been successful: yet still there has been a ne plus ultra, a certain barrier by which their investigations have been limited, which no person has hitherto been able to pass, and which it is very improbable ever will be passed."

That unknown property by which a muscle when wounded, touched or irritated, contracts, independently of the will of the animal that is the object of the experiment, and without its feeling pain, is called by Haller, its vis insita, or inherent power.

Now it is to be observed that the vis nervea, or nervous power, which Dr. Monro, in accounting for muscular motion, contrasts with the vis insita, comes to the muscle from without, that is, from the influence of a brain and nervous system; whereas the vis insita resides constantly in the muscle itself. The nervous power ceases when life is destroyed; the other appears, from experiments, to remain for some time. after death; the nervous power is also suppressed by tying a ligature upon the nerve, by hurting the brain, or by taking opium. The vis insita suffers nothing from all these circumstances; it remains after the nerve going to the muscle is tied; it continues in the intestines, though they be taken out of the body and cut in pieces; it appears with great strength in such animals as are destitute of brain : that part of the body is moved which has no feeling; and the parts of the body feel which are without

motion. The will excites and removes the nervous power, but has no power over the vis insita.*

Sir Gilbert Blane in his Croonian Lecture, delivered in the year 1788, considers, in the course of his reasoning, that the nervous system is not only a mere appendage to life, or muscular irritability, but that it tends to impede its operation, and to shorten its existence. Hence he maintains that muscular. irritability does not depend upon a sentient principle. Many animals, it is well known, exist without brain or nerves. This was first observed by Haller, and was confirmed by Hunter: who maintains farther that the stomach is the centre or seat of life, and more essential to it than the brain. That the stomach should be an organ of so much consequence, seems natural enough, from the importance of its function, which is that of assimilation or nutrition; and life can be more immediately and completely extinguished by an injury done to it, such as a blow, than by the same violence to any other part of the body. It is also well known, as before observed, that the muscular fibres of animals endowed with a nervous system, will retain their irritability for some time after their separation from the brain and nerves. And from the phenomena of vegetation, he thinks irritability may exist in nature, without sensation, consciousness, or any suspicion of the existence of a nervous system. Besides, those animals which are

* Edinburgh Medical and Surgical Dict.