changes to yellow or red, as we see in sickly or dying leaves.

Where plants have lost their colour, from being placed in darkness, they contain a smaller quantity of principles soluble in alcohol than when in a green state, as has been shown by Senebier in his Physiologie Végétale, vol. iv. p. 278. Hence, principles deprived of oxygen which are soluble in alcohol are produced only under exposure to light.

The colour of flowers depends upon their degree of disoxydation, and may be arranged thus: 1. red changing into blue; 2. blue; 3. yellow; and, 4. scarlet, which is a modification of yellow: green is very uncommon.

Fruit is at first green: it is afterwards either scorched by dryness, or, by the retention of oxygen, becomes sweet or acid.

Mineral colours are known to depend upon particular proportions of water: as, for instance, sulphate of copper. Can they depend in plants upon a similar cause; that is to say, upon particular proportions of oxygen and hydrogen?

The odours of plants depend upon a certain volatile oil, which varies extremely in different species. It is more or less volatile, more or less soluble in alcohol, and more or less miscible in water; on which accounts there is a great difference in the readiness with which it yields to distillation. This is especially apparent in the oil of roses, which, in hot countries, or in hot weather, passes over very readily; but, in cold seasons, inseparably combines with the water used in distillation.

There is no doubt that particles of the odoriferous oil are continually flying off; and it has even been asserted, that those of Dictamnus albus will inflame upon the application of fire.

In proportion to the more or less volatile nature of their oil are the odours of plants more or less diffused. Hence in the same plant several scents may be distinguished, some at a distance, some on a near approach, as in the flowers of Vicia Faba. Some odours are not perceptible until the parts of a plant are rubbed, or they are more copiously given out when this is done. Some are most apparent in the recent state,

others in the dried, as is the case with the melilots: in these the odoriferous juices are combined with too much water, which it is necessary to separate by drying before the fragrance is apparent.

Some flowers scatter their fragrance only at night: this is particularly the case with Cruciferæ, having dingy yellowish brown petals. In these the volatile oil is either so rapidly dispersed in the day-time as to be imperceptible; while it is more slowly and densely evolved in the night: or it may be decomposed in the day-time by the extrication of oxygen.

Plants in hot countries are more fragrant than those of cold countries, as is apparent in the rose; but their fragrance is sometimes so much dispersed by heat as to be imperceptible. Nocca found the Calendula lose its smell in a hot-house; and the common horehound is destitute of smell in Portugal.

The tastes of plants are sweet, acid, bitter, astringent, or austere and acrid. Vegetable membrane and resins communicate no taste to the palate, as they do not dissolve in the mouth. Mucilage has no flavour; sweetness is derived from the presence of sugar, and acidity of acids; bitterness usually denotes the presence of extractive matter. The extractive principle may be compared with neutral salts, which are usually bitter, as sulphate of potash, lime, and magnesia combined with acids and others; for, as a neutral salt consists of a metal, inflammable matter, and oxygen, so extractive matter consists of carbon, hydrogen, and oxygen. The astringency of minerals is only perceptible in those neutral salts that contain acid in excess, as alumine, sulphate of iron, sulphate of copper, and others. With these we may compare the astringent principle of vegetables, which also contains acid in excess; for it not only combines with the earths, but also stains litmus paper red: the carbon and hydrogen it contains may be compared with a metal and combustible matter. That kind of taste which is called herbaceous is caused by a mixture of mucilage with a little astringency. An acrid taste is almost peculiar to volatile oils,

CHAPTER XI.

OF THE DIRECTIONS TAKEN BY THE ORGANS OF PLANTS.

THE substance of all that is known upon this subject has been combined with some excellent observations of his own by Dr. Dutrochet in a memoir, of which I shall avail myself in the following remarks:

"The general phenomena of nature," says this writer, "which are daily before our eyes, are often those which mankind considers the least attentively. Those who are unaccustomed to reflect upon such subjects can scarcely believe that there is any very extraordinary mystery in the ascent of the stems of vegetables, or in the descent of their roots; and yet this is one of the most curious circumstances connected with vegetable life. The downward direction of the roots may appear easy of explanation: it may be said that, like all other bodies, they have a tendency towards the centre of the earth, in consequence of the known laws of gravity; but on what principle, then, is to be explained the upward tendency of the stem, which is in direct opposition to those laws? and here lies the difficulty. Dodart is the first who appears to have paid attention to this circumstance: he pretends to explain the turning backwards of seeds sown in an inverted position by the following hypothesis: he assumed that the root is composed of parts that contract by humidity; and that the stem, on the contrary, contracts by dryness. For this reason, according to him, it ought to happen that, when a seed is sown in an inverted position, the radicle will turn back towards the earth, which is the seat of humidity; and that the plumula, on the contrary, turns to the sky, or rather atmosphere,—a drier medium than the earth. The experiments of Du Hamel are well known, in which he attempted to force a radicle upwards and a plumula downwards, by enclosing them in tubes, which prevented the turning back of these parts. It was found that, as the radicle and plumula could not take their natural direc

tion, they became twisted spirally. These experiments, while they prove that the opposite tendencies of the radicle and plumula cannot be altered, still leave us in ignorance of the cause of such tendencies. We are equally ignorant of the

cause of the directions of the leaves. Bonnet believed that he could explain that phenomenon upon the hypothesis of Dodart just referred to, with respect to the radicle and plumula. According to him the lower surface of the leaves is, like the radicle, composed of fibres which contract by humidity; and the upper, like the plumula, of fibres that contract by dryness. As a proof of these assertions, Bonnet manufactured some artificial leaves; the upper surface of which was parchment, which contracts by dryness, and the lower of linen, which relaxes by moisture. These leaves were submitted to the action of dryness and humidity; and Bonnet found they were affected much in the same way as true leaves,—so easy is it to find proofs to support a favourite hypothesis."

In consequence of the unsatisfactory nature of these and other theories, more modern physiologists have been satisfied with inscribing the particular directions taken by plants among the vital phenomena of vegetation. And this is, perhaps, as much as we are likely to ascertain relating to it, and all similar manifestations of the overruling power of nature. Dutrochet, however, being of opinion that some more direct explanation of this phenomenon is to be found, instituted a variety of experiments of a novel kind. "Seeing," he remarks, "that the stem is always directed towards heaven, and the roots towards the earth, we cannot but believe that there is some relation between the cause of gravitation and that of the life of vegetables: the constant direction of the stem towards the light leads us also to suppose that this agent performs some important part in determining the directions of the parts of plants. The stem must be placed in the midst of the atmosphere in order to develope itself; the roots, on the contrary, require to lie within the earth. Hence, it may be inferred, that several causes concur to produce the phenomena in question."

Dutrochet filled with earth a box, the bottom of which was perforated with many holes: he placed seed of the kidney

bean in these holes, and suspended the box in the air, at about eighteen feet from the ea. Here the seeds, being placed in holes pierced through the bottom of the box, received the influence of the atmosphere and light from below; while the humid earth was placed above them. If the cause of the different directions of the radicle and plumula consisted in an affinity of the former for humidity, and of the latter for the atmosphere, the radicle ought to shoot upwards, and the plumula downwards; but this did not take place. The radicles, on the contrary, found their way downwards out of the box into the atmosphere, where they quickly dried up and perished; and the plumulas forced their way backwards into the earth. This experiment was afterwards modified, by increasing the quantity of earth above the seeds, and by some other contrivances; but the result was always the same: it was uniformly found, that there was no affinity between the radicle and the seat of moisture sufficient to counteract the natural downward tendency of the roots. was also inferred, that there existed no more positive affinity between the stems and the atmosphere than between the roots and water.

It

There are certain parasitic plants which strike their roots into the stems of other plants, and which always grow at right angles with the stem to which they are fixed. The seed of the miseltoe will germinate in any direction, either upwards, downwards, or laterally. The first movement made by this plant consists in an extension of its cauliculus, which derives its support from the cotyledons, and which terminates at the radicular end, in a small green tubercle of a paler colour than the radicle itself. When the seed is fixed upon a branch by its natural glue, this incipient movement is effected at right angles with the branch; the young shoot is then curved backwards, and the radicular extremity descends to the surface of the branch, to which it adheres by expanding into a kind of disk. From this expansion the roots are emitted, and penetrate the interior of the branch whereon the seed of the miseltoe is fixed: its stem takes the directions above mentioned with reference to the centre of the branch on which it is fixed, and not with reference to the earth; so that,