quirer is, that though the mass of fluids from which the secretions are produced be one and the same, and the secreting organs, as to externals, be often in the same condition, yet the matters secreted in these various organs differ greatly from each other. Thus, an essential oil is found only in the rind of the orange, a fat oil only in the kernal of the almond; and so with regard to other secretions, which exist only in particular parts. Besides the acids, alkalis, earths, and metals, which, though of a mineral nature, are more or less constantly found in plants, chemists enumerate about forty products of vegetation, which possess distinct chemical characters; and of many of these products numerous varieties exist. As none of these substances can be detected in the common sap, they must have been elaborated by the specific organs of vegetables, under a process of secretion. By what peculiarity of structure or of function these organs are enabled to produce such remarkable chemical changes in the common sap, is quite unknown; neither do we know how much is to be attributed to the action of the organ itself, or the reaction of the several ingredients on each other, or to the influence of external agents. From all that appears, however, these changes are necessarily connected with the principle of life, no such effects being ever produced either by experiments on the sap itself, independent of the secreting vessels, nor by any extraneous combination of ingredients.*

Of all these products of secretion, the most remarkable, and the most important to the plant, is a mucilaginous fluid named Cambium, which is employed directly in vegetable nutrition and growth, and may be

* It is a curious fact, that some of the substances most essential to the nutriment of plants, though differing very materially in their sensible qualities, are, on chemical analysis, found to be composed of precisely the same constituent parts, united in proportions but slightly differing from each other. Thus, Dr Prout gives the following analysis of the relative composition of gum, starch, sugar, and lignim :

considered as analogous to chyle in animals. This substance is formed directly from the proper juices, which, again, are formed from the sap; and though these differ from each other very widely in different plants, as to their sensible and chemical properties, the cambium seems, in all plants, with regard to these, to be nearly the same. It is without colour, odour, or taste, though the proper juices exhibit all these qualities. The proper juices, also, are contained in the vessels, and flow out when they are divided; but the cambium transudes rather than flows, and that only in places where new parts are to be formed. Thus, in the pine, says Mirbel, while the proper, or resinous juice, flows in the large vessels, the cambium transudes beneath the liber. Of the other secretions of plants, which are found chiefly on the external parts, such as the leaves, flowers, fruit, &c., the number and diversity are very great, some of them calculated to delight the sight, the smell, or the taste; and others of the first utility in the arts, or in contributing to the subsistence or comfort of animal life.*

This slight notice of the phenomena of secretion, is sufficient to fill the mind with wonder and devout admiration in reflecting on the attributes of the Creator, exhibited in this familiar but most mysterious operation, by which the surface of the earth is replenished with food fit for the nourishment of living beings, adorned with beauty, and rendered fruitful of blessings. First, the sap is collected from the crude earth, consisting of common water, holding in solution a few well-known substances, which the soil abundantly supplies. This fluid is then carried through capillary tubes to the leaves, where it is, by some obscure chemical process, in which the light, heat, and gasses of the atmosphere, combine their secret powers, converted into a peculiar juice, corresponding to the constitution of each species of plant, and, in different species, possessing different and some

* Supplement to Encyclopedia Britannica,-Article, Vegetable Physiology.

times directly opposite qualities; such are volatile and fixed oils, resins, gum-resins, balsams, turpentines, tannin, and extractive, with alkalis and acids, earthy and saline compounds, &c. From this peculiar juice, again, are secreted substances possessing apparently none of its characteristic properties, which perform various important functions, and one of which is destined to form the particular tissue of the plant that exudes it, and to become, indeed, part of its substance, thus, at one time moulded into the gnarled limb of the oak, and at another into the tender and pliant twig of the willow.

I cannot, without deviating too far from the popular plan of this work, enter more minutely into the physiology of vegetable substances; but there is one wellknown effect of the processes of vegetation which I may just allude to, in order to call the reader's attention to a very remarkable adaptation with which it is connected. Dr Priestley made the discovery, that oxygen gas is exhaled from the leaves of plants during the day; and the investigation which he commenced has since been continued by others, the result of which is, that a species of respiration in plants has been ascertained, depending on the alternations of day and night, which is essential to their vital powers, the oxygen exhaled during the day being inhaled in the night. This affords us a new analogy between vegetable and animal existences, which is not a little curious; but let it be observed, that the object to be answered by this vegetable respiration, is exactly the converse of that which is answered by the breathing of animals; in the former, it is that of adding carbon, in an assimilated state, to the plant; in the latter, it is that of discharging the superfluous quantity of carbon from the animal.

There is something exceedingly worthy of remark in the fact just mentioned, as it opens up a very unexpected, but most interesting view of the kind of action and reaction which goes on between the vegetable and animal creations, and of the salutary balance which is

thus kept up in the two organized kingdoms. The oxygen exhaled during the day, completely compensates for that which was inhaled during the night; and, in whatever degree the atmosphere has been deteriorated, in its capability of sustaining animal life, by the discharge of carbonic acid from the lungs of animals, the demands of vegetable life for that injurious substance, tend again to restore it to a healthful state. "The oxygen given out by plants," says Dr Roget, "and the carbonic acid resulting from animal respiration, and from the various processes of combustion, which are going on in every part of the world, are quickly spread through the atmosphere, not only from the tendency of all gasses to uniform diffusion, but also from the action of the winds, which are continually agitating the whole mass, and promoting the thorough mingling of its different portions, so as to render it perfectly homogeneous in every region of the globe, and at every elevation above the surface."

Looking at the whole subject of vegetable physiology, even in the very imperfect outline of it which has now been traced, it is impossible not to recognize the Designing Hand of infinite power, wisdom, and goodness. The processes are so astonishing, the results so beneficial, and the adaptations so skilfully adjusted, that the man must be wilfully blind who does not see, in them all, the presence of a Father-God. Were but one of these to fail or be deranged, disorder and destruction would inevitably ensue. And yet most of the changes and combinations of organic life are of the most delicate nature, depending upon conditions which a very slight deviation on either side would render nugatory; thus reminding us, continually, of the wisely regulated balance which the Creator has established, and sedulously preserves,—and teaching us the salutary lesson, of constant dependance on His providential care.

THIRD WEEK-TUESDAY.

FLOWERS. THEIR FORM, COLOUR, AND FRAGRANCE.

THE variety which exists in the vegetable world is an example of a quality which pervades all nature, and stands forth in each of its departments as a very conspicuous feature in the character of creation. There is a continued chain of existence, commencing with the most crude materials, and passing from earth, rocks, and metals, to the more subtle elements which compose water, air, and light; and thence again to vegetable productions, rising through the various tribes of mosses and fungi, to grasses, shrubs, and trees, till Nature combines all that is beautiful and delightful in this department of her works, in the formation of flowers. These, though comparatively minute productions, yet contain in their construction, both as regards its appearance and its uses, so many proofs of beneficent intention, that there seems no part of the vegetable world which presents, in so concentrated a form, such varied evidences of a Divine Hand.

"Flowers may be regarded not only as the last, but the most elaborated organs of the vegetable system. Whether we contemplate the beauty of their forms, the splendour of their colours, or the delicious fragrance they every where breathe around us; or whether, with a physiological eye, we survey the delicacy of their structure, and investigate the peculiar functions they perform, we cannot but feel the greatest admiration of the skill with which, in a compass so small, and by means apparently so simple, such a series of actions, terminating in results so varied and important, can at once be combined and regulated."* In this short but comprehensive description, two intentions of Creative Intelligence are indicated, the object of the one being to

Supplement Encyclopedia Britannica, Art. Vegetable Physiology.