both the natural and the moral light to the capacities of his creatures to receive it. Our eyes would not bear to be nearer the sun, and our intellectual sight could not endure the full revelation of celestial glory. This St. Paul experienced when he was caught up to the third heaven, and "heard unspeakable words, which it is not lawful," or rather, according to the marginal reading, it is not possible for man to utter" (2 Cor. xii. 4); and what it was not possible to communicate, we may infer it was not possible, without a miracle, to receive. We are not, therefore, to presume that moral difficulties cannot be cleared up; much less are we to suppose that they are arguments against the truth of divine revelation, or the perfection of the divine attributes; but rather we should conclude, that if any explanation were to be given, we could not, with our finite faculties, comprehend it; in fact, that the defect is in ourselves, in our contracted and consequently mistaken views, and not in the arrangements of an almighty and all-wise Providence.

But, further; the clearing up of all difficulties would be unsuitable to our present probationary state, inasmuch as it would give no scope for the exercise of the moral discipline which we need. If all the doctrines of religion were forced upon us by irresistible evidence, we should receive them, not because God hath revealed them, not upon his authority, but because we could not withhold our assent to what had been clearly proved to us. Now here would be no trial of faith, no test of obedience, no room for filial confidence, consequently no means for the formation of those important principles which constitute the Christian character: but all would be cold, calculating demonstration,-a religion of the intellect, not of the heart-a religion which would give light indeed, but not heat, for it would leave the motives, the affections, and the principles untouched. Who does not see that this speculative scheme would be inconsistent with the state of probation or trial, which we evidently require, or God would not have placed us in it? for there can be no trial without difficulties, and there can be no difficulties where all is plain and clear to us: -at the same time, it would be totally inadequate to the improvement of our moral faculties; for on this purely intellectual scheme there could be no inculcation of those holy feelings of faith, hope, and charity, which Christianity is so eminently calculated to call forth.

That almighty God is willing to impart spiritual light as far as we are able to bear it, and in the manner and degree which will be best for us, we may learn from his past dealings with mankind. When our first

parents had yielded to the temptation of the evil one, and suffered the eyes of their understanding to be darkened, so that they could no longer enjoy the brightness of the Divine presence, a glimmering light, just enough to keep hope alive, was vouchsafed in the promise that the seed of the woman should bruise the serpent's head; and this light gradually increasing, as man was able to bear it, shone with a growing though mysterious lustre under the veil of the ceremonial law, an emblematical dispensation adapted to the infancy of the human race; and yet, again, as time rolled on, this light became more clear and distinct, when the prophets pointed to the rising of the Sun of Righteousness, till at length the day-star stood over the manger at Bethlehem, and that light which lighteth every man beamed upon a benighted world.

Now let us not be guilty of the egregious folly of rejecting the light of divine revelation, because we have not more of it. The traveller does not put out his lamp in a dark night, because he cannot have the light of the sun, but he makes the most of what he has, and carefully walks by its guidance; and so we should do well to "take heed" to that light from heaven which shineth in a dark place, cherishing it, watching it, following it; for though it be but a taper, compared to that flood of light which God, if he saw fit, might pour upon us, let us remember that it is the only light we have to guide us through this dark world. Nor let us indulge in idle lamentations that more is not vouchsafed to us; for we may be assured, that God has made every thing suited to its purpose,

good after its kind;"-we have enough for our safety, if we will but take heed to it; more might only bewilder us. We have enough to guide us to our journey's end; if we had more, it might only allure us out of the way, and induce us to linger over the barren fields of unprofitable speculation; we might be for clearing up this doubt and that difficulty, instead of taking heed to our ways, that our footsteps slip not, which, after all, is the great practical purpose for which we need light.

For this end-to fix our attention on practical points-it doubtless is, that we are always disappointed when we search the Scriptures to gratify mere curiosity; in vain do we interrogate the sacred oracles on such subjects-they maintain a portentous silence, or the only response returned is, "Secret things belong unto the Lord your God." Or do we impatiently press the matter further, and pry into the new dispensation, in hopes of discovering what is denied to us in the old, we are meet with the mild rebuke of our

Lord, "What is that to thee? follow thou me." And if we receive the rebuke as we ought, and endeavour humbly to follow our Lord, by taking heed to the light he has vouchsafed to us, we shall, as we advance, perceive it shining more and more upon our path, till the dawning gleam of holiness and the morning-star of purity will usher in that resplendent day, of which it is said, "There shall be no night there: and they need no candle, neither light of the sun; for the Lord God giveth them light, and they shall reign for ever and ever."

WHEN the stem of a plant is designed to be of a greater duration than one or two years, a provision must be made for its progressive increase in size and solidity. Why is this necessary, and how is it accomplished? It is requisite, because in vegetable structures the same part rarely serves a second time for the fulfilment of its object, it being one of the leading distinctions between animals and vegetables, that the former are nourished by a constant renewal of all parts through the energy and combined action of the whole; while in plants, parts once completely formed, and which may be regarded as the scaffolding or ground-work of subsequent structures, become merely rigid, and are not again dissolved, but augment in thickness and opaqueness, so that nutrition can only be accomplished by the continual formation of fresh elementary parts, or organs. Thus it is that an animal, having reached maturity, remains stationary in size, having a definite form according to its kind, and only exhibiting variations within certain limits; a tree, on the opposite hand, goes on increasing in height and the extent of its branches year after year, till it covers a large space of ground. The rate of increase diminishes in all species of trees after a certain period, from a variety of causes; but still, so long as the tree is a living structure, some new growth occurs, and in the winter-season it is only at its minimum, a slight increase happening even then-for the complete cessation of this process in a tree is equivalent to its death. Hence the visible addition to the size of trees, which is one of the most familiar phenomena of nature.

All the functions or actions of plants are begun or excited by heat, and completed by light; when, therefore, a seed (in which a germ or embryo, that is, a new plant on the most reduced scale, is contained) is placed under ground, when the temperature is sufficiently high, it is stimulated to the development of its miniature organs; one set penetrating deeper into the earth, the other rising above it. The ascending shoot, or plumule, is slender, generally pale or very light green, and scarcely capable of standing erect. bring not with it above ground the cotyledonary leaves, or seed-lobes, its growth is for a time arrested, a thickening of its summit occurs, and at this point,

If it

after the upward development is resumed, a leaf will be found to be formed, and which will project from the axis in a direction more or less horizontal. The point where the thickening occurred is termed a nodus, or joint; and, owing to the connexion which subsists between the stem and the leaf attached to it, a portion of the crude sap collected by the extending root is Being thus diffused over a large surface, which is also conveyed into the tissue of which the leaf consists. thin and permeable by the air, it is freely exposed to the action of air and light. The horizontal position of the leaf allows the solar ray to impinge upon it, and its delicate texture permits the chemical rays to reach the fluid distributed through the cells. This fluid contains carbonic acid in various states, and the leaf has besides the property of absorbing carbonic acid gas from the atmosphere. This carbonic acid consists of oxygen and carbon, or charcoal. The advantage of light to the plant arises from its peculiar power of removing oxygen; and when the direct solar ray falls on the leaf, the carbonic acid is decomposed, i. e. resolved into its constituent elements, the oxygen being thrown into the atmosphere, and the carbon retained by the plant. By the vital action of the vegetable tissues, aided by the light, fresh combinations are formed; and though the ultimate products of plants differ, owing to essential differences in their structure, also age, soil, climate, and other causes, yet their first actions seem limited to uniting the carbon with water, so as to form a class of compounds termed hydro-carbonates, viz. gum, sugar, starch, and lignin. According to chemical analysis, 100 parts of

Of these, the latter requiring more carbon for their formation, need a longer exposure to light to accomplish this combination; so that all young plants, or recently formed parts of old plants, are generally gummy or sweet. The ear or head of wheat, as well as the young stalk or shoot, is at first extremely sweet, a quality which these parts ultimately lose when exposed sufficiently long to the solar ray to permit the additional quantity of carbon to be fixed, which converts sugar into starch. In plants of a woody texture a still longer exposure of the leaves to light enables lignin to be formed, which gives stability and durability to plants; and the more powerful the solar ray, the more rapidly and efficiently is this combination brought about. Hence the greater density as well as deeper colour of woods of tropical countries, compared with those of temperate and northern climes.

From containing merely gum or sugar, young plants are not only softer and more easily digested than old ones, but even poisonous plants are generally innoxious at an early period, and may be eaten with impunity. The same is the case when the light is excluded, for then the stem is feeble and creeps along the ground (as may be observed when potatoes sprout in dark cellars), or in the process of blanching adopted by gardeners, by which celery and lettuce are kept soft and wholesome, which, if freely exposed to light, would become tough, thready, and poisonous.

To return to the progressive development of the stem. A portion of the nutritive materials elaborated by the leaf returns through a set of vessels with which it is provided into the stem, there contributing to its support and consolidation. A corresponding, or even greater, extension has taken place in the root under ground, by which additional crude sap is absorbed. To elaborate this, more green leaves are needed. The upward growth of the axis is resumed, a space intervenes between the first node or joint, and one which will be formed on a similar plan, with its leaf attached this space is termed internode.

It deserves to be remarked, that the leaf of the second node is not exactly above the former, but a little to the one side. This process is repeated several times in succession, the leaf being in each instance placed somewhat laterally with respect to the preceding one, till at last one is found to be placed perpendicularly over the first leaf. The intervening leaves constitute a helix, or spire; and if the temperature has not fallen by the advanced state of the season, from the time occupied in this growth, a repetition of what has been last described will occur, so that two, or in trees of very rapid growth, three or more spires of leaves shall have been formed before the winter repose takes place.

From these consecutive developments of vegetable tissue, a simple stem or rod devoid of branches results. The provision for the growth of the subsequent year, and for the formation of branches, takes effect in this wise:-The upper point of the axis is occupied by a bud, which, from its position, is designated terminal bud, and which, but for the reduction of temperature, would have been developed the first year, adding then another internode to the length of the stem; but the cause just assigned having retarded its growth, it is necessarily postponed till the rise of temperature the following spring. The result of its development, however, would only be to elongate the axis; the branches found proceeding like so many radii from the centre, during the second year, originate in buds termed lateral. Wherever, during the first year's growth, a nodus was formed, and a leaf developed, in the axil of that leaf a vital point, or leaf-bud, was also formed. When, at the conclusion of the autumn, the leaves are detached from the stem, the leaf-buds remain. "As a seed is the rudimentary state of the entire plant, so is a leaf-bud of the branch. The origin of these buds is one of the mysteries of physiology, which it appears to be as far above the power of human knowledge to explain, as it is to account for the creation of an entire organised being. But the cause which operates as a stimulus to their development in one part of the vegetable structure rather than in another, is probably owing to an accumulation of nutriment in that part, originating in some check which is there given to its general diffusion through the system. Thus the nutriment prepared in the leaf may be supposed to meet with a check at the axil, in its course into the stem, and the immediate consequence is the increased activity of one or more of those latent germs of vitality from which a leaf-bud results."

Before considering the results of the development of these lateral buds and the diverging branches, it is necessary to examine the internal structure of the portion formed during the first year.

When a transverse or horizontal section is made of the young stem, there is observed a centre of pith with a thin circle or zone of wood around it, exterior to which is a zone or circle of bark. The figure of the entire stem or shoot is conical, which is owing to the circle of wood being thicker near the base than towards the apex. This greater diameter is occasioned by the leaves having sent from the base of each a number of fibres of wood, and as the leaves diminish towards the summit, the thickness is greatest below. The pith consists of vegetable tissue termed cellular, radiations from which pass through the stratum of wood, dividing it into wedge-shaped pieces, and so maintain a communication between the pith and the bark; originating in the pith or medulla, they are termed medullary rays. The wood during the first year, and in most trees for several years after, is of a light colour, hence called alburnum, and of little den

Henslow, in the Guide prefixed to "The Botanist." Το Mr. Maund, the conductor of that beautiful work, I have to express my obligation for the use of the woodcut illustrating this paper.

sity or durability. But in the greater number of instances it ultimately acquires a deeper colour and greater density, when it is termed duramen, or heartwood.

Such is the state of things at the commencement of the spring of the second year. Every part of the plant is soft, every cell or vessel clear and easily permeable by fluids, especially by the new or crude sap to be transmitted to the stem by the renewed activity of the roots. Instead of having a seed under ground, as at first, with only one germ or vital point, we have above ground an elongated axis, on which are found numerous buds, one terminal and several lateral. These are strictly analogous to seeds, with this exception, that a seed germinates best in the dark, (for reasons which can be best explained when the structure and chemical composition of seeds shall have been described); buds develop themselves most readily in the light. The increased temperature of spring, with the increasing intensity of the solar light, stimulates these

buds to action.

The terminal bud, from its more elevated position, and being in a more direct line with the base of the stem, along which the ascending sap flows, not only generally precedes the others in the order of development, but, in almost all instances, makes a longer shoot or greater growth in the course of the season; thus extending or elongating the axis, from which subsequent branches are to proceed. So great, indeed, are the advantages of position of the terminal bud, that it would absorb the whole sap, and form one long shoot, completely robbing the lower ones, were it not for a contrivance which must now be explained. Where the arrestment of growth took place during the first year, and wherever it occurs during all the subsequent years, for the formation of a nodus, with its leaf and leaf-bud, there a peculiar interlacement of the tissues of the plant happens; a process or projection from the pith, extending into the lateral bud, being the result, so that each lateral bud is as perfectly connected with the pith as the terminal bud itself, which may be looked upon as the extremity or summit of the pith: but to effect which for the lateral buds, a deflection of the fibres descending from the buds or leaves above is occasioned, (as may be at once seen in a knot of wood, which has the curved state of its fibres occasioned by the passage required for the pith,) by which the ascending sap is for a time delayed in its course, and a portion of it forced off laterally for the nourishment of the bud in the first instance, and to be aërated and exposed to the light in the leaves with which each branch is clothed, precisely like the perpendicular shoot or stem. The system of distribution of the sap through the lateral branches is as complete as that accomplished in artifical systems of irrigation, by which the main channel being temporarily closed, and the side-sluices opened, the entire territory receives an equal share. That this is the primary use of the nodi can be shewn by an appeal to the stem and habitudes of endogenous plants, such as palms, which are to be presently detailed; but, with that consummate skill which reigns through all the works of the supreme Artist, we shall find that they serve another use in arresting the descent of the elaborated sap, and forcing it off laterally into the flower-buds and fruit. Thus they not only produce their first end,

"But serve to second too some other use."

The consequences of the renewed growth of all parts, found existing at the end of the first year, at the end of the second season are, the increase in height, and the projection from the axis of radii or branches, each provided with its leaf-buds, in reserve for the following spring, the development of which will produce, in respect of every branch, the same effects that the development of the lateral buds had on the primary axis or stem. Each branch has its terminal bud, the

growth of which will lengthen the radius, while the development of its lateral buds will constitute so many radii around the secondary axis.

The condition of the internal parts is now to be examined. From the point whence the upward growth was resumed to the summit, an exact counterpart of what was found in the stem of the first year exists; but below that point will be found two cones of wood, one overlaping the other, so that a horizontal section exhibits a centre of pith and two zones of wood, exterior to which is the bark; and a close inspection of that will likewise shew that a thin stratum has been added to its internal side. The third year similar phenomena occur, so that there are then three strata of wood at the base, two above, and one at the upper part. Up to this period, and even later, the pith will be in general found precisely in the centre, so as to constitute at once the botanical and mathematical centre of the tree. If, however, the tree have stood in a field or park, so that no external interference, such as the proximity of other trees, as is the case in crowded woods, shall have influenced its development, rendering it difficult in one direction, but easy in another; and if at the end of twenty or twenty-five years a horizontal section of the stem be made, the pith will be found to be eccentric, that is, though still the botanical centre of the stem, it will not be the mathematical centre, owing to the annual strata of wood being thicker on one side than the other. The side of the tree which was exposed to the south and the south east (in northern latitudes) will be found of considerably greater diameter from the thickness of the zones on that side, than that which faced the north and north-west. The beautiful symmetry which characterised the tree while a sapling is lost, and the branches will be found not only more numerous, but longer on the one side than on the opposite. The superiority in number and length will, in a solitary tree, always be observed to be on the south-east, provided the soil was of uniform quality around the stem. The cause of this must now be explained. The buds of plants, it has been already stated, are stimulated to growth by light; the side exposed to the south-east and south receives a greater amount of light, i. e. not only during a greater number of hours, but generally of a greater intensity; and more buds are therefore developed on that side. The consequence of this is, that more leaves are spread out on the side of the plant where, by the greatest increase of surface, the greatest advantage will accrue to the tree. Thus we find the power which caused the development of the buds, operating to ensure the utmost augmentation of surface where the most beneficial effects will result. This is one among the number of striking instances which the works of creation afford of the end being obtained with the greatest economy of means. More diversified agencies might have been employed, and

the object attained as effectually; but that simplicity, so characteristic of the operations of the Deity, would not have prevailed, to arrest the attention and impress the minds of his intelligent and reflecting creatures.

The condition of the opposite side of the tree merits observation. On it, in the majority of cases, will be found a greater number of cryptogamic or cellular plants, such as mosses, lichens, and jungermannias, than on the other. These plants can vegetate under a fainter light than vascular plants, and flourish best where considerable humidity exists. The northern side of the tree is therefore most suitable for them, and their prevalence on that, in preference to the southern, furnishes a guide not only to the savage when tracking his way through the forest, but travellers have been enabled to regain their lost route in many instances by attending to this sign, which thus served them instead of a compass.

The seed performs its office in supplying the first shoot; all subsequent shoots are derived from buds; their importance is therefore obvious; and it would only be consonant with the general care which is manifest throughout creation to find special provision for insuring their safety, particularly during the winter. Accordingly the buds of deciduous-leaved plants of cold countries are in general wrapped up in certain rudimentary leaves or scales, so folded over the tender point in the centre as to defend it from cold and wet. These scales are always definite in number, and arranged according to a uniform plan, not only in every bud of the same plant or tree, but in every individual of the same species. Their form, position, and number, not only completely encases the important vital part within, but they are frequently coated with a resinous or glutinous and insoluble juice, which renders them impenetrable by rain, and often have a soft down or woolly coating internally, which preserves the warmth of the bud. The buds of the horse-chestnut (Esculus hippocastanum) have both the resinous juice externally and the down within; several poplars, such as Populus balsamifera, willows, &c., are either glutinous or woolly. So long as these scales remain closed, the growing-point is safe, even under a great reduction of temperature; and when the return of warmth in spring stimulates them to action, we uniformly see them "burst their cerements and awake." It deserves to be stated (more especially as one object of these papers is to shew that the utmost precision is displayed in the organisation of plants), that each kind of tree has a definite number of scales in every bud, and that the shoot which results from the development of all of them never possesses beyond a certain maximum of internodes. Thus, the ash (Fraxinus excelsior) has in each bud two pairs of scales, five pairs of leaves, and, at the utmost, three internodes in each shoot of the same year; the horse-chestnut has seven pairs of scales, five pairs of leaves, and five internodes; the Acer campestre (or field-maple), six pairs of scales, five pairs of leaves, and ten internodes; the Sorbus (pyrus) àucuparia, three pairs of scales, five pairs of leaves, and eight internodes.†

Such are some of the peculiarities of structure of the trees of high and temperate latitudes, which are of the kind termed exogenous. We must now direct attention to those of tropical and subtropical latitudes, many of which are of the kind termed endogenous. When a seed of a palm (which is one of the most convenient examples) begins to germinate, one leaf appears rather external than above the first,

"He marks the bounds which winter may not pass,
And blunts his pointed fury; in its case,
Russet and rude, folds up the tender germ
Uninjured, with inimitable art;

And, ere one flowery season fades and dies,

Designs the blooming wonders of the next."-Cowper.

+ See Ohlert, Einige Bemerkungen über die Knospen unserer Bäume und Straucher,-in the Journal called "Linnæa," 1837, pp. 632-640.

the increase being horizontal more than perpendicular, till suddenly from the centre there shoots up a stem, of a nearly cylindrical form, from which, as well as the diameter it possesses at first, it scarcely ever deviates. The tendency of these stems is rather to increase in length than breadth, and the new matter is added to the interior of the cylinder, which is originally hollow, or of a loose texture, the old matter being pressed towards the sides. Thus these stems increase in solidity, the hardest portion being outwards; and, if they are not extremely slender, they can stand erect even when 110 feet, though only three inches at the base; if very slender, they trail over the tops of other trees, sometimes even being 300 feet long and not half an inch in diameter, such as the Calamus rotang, or cane. In these plants the terminal bud alone is developed in general, the whole of the ascending sap being consumed by it, the lateral ones not having the advantage of nodi, as seen in the exogenous stems; the consequence of this is, that the stem of a palm is a simple branchless cylinder, with a tuft or rosette of leaves at the top. Grasses, such as the bamboo, have nodi or joints, and also lateral leaves and branches, a fact by which they not only approximate to exogens, but which demonstrates the use and office of nodi. In some palms, such as the doom-palm of Egypt (Cucifera thebaica),* two terminal buds on the same plane uniformly develope themselves, and give rise to a stem presenting a succession of forks, or a dichotomous stem. The internal structure of endogenous plants never exhibits the stratified character of exogenous stems, there being no pith with concentric zones around it. The peculiar adaptation of these stems, with their solitary bud at the apex, to the climes where they predominate, becomes manifest when we reflect upon the direction of the rays of light between the tropics, which is not only more perpendicular,† but the rays invariably fall upon the stems on two opposite sides in the course of each

season.

It is impossible for the European to imagine the effect on the scenery of these chiefs of the vegetable kingdom, whether they grow in masses, or present solitary stems towering above the other mighty trees of the equinoctial regions. The foliage is often of the same gigantic character as the stem, and one trunk, with its majestic crown of leaves, affords shade and shelter to the weary traveller sinking under the scorching sunbeams, which yet are the means of bringing to perfection these gorgeous attributes of tropical climes.

The fluids which, collected by the roots, permeate the more superficial layers of exogenous plants, especially of the leaves, would escape by the rapid evaporation, were it not for an arrangement which requires to be noticed here. Organised structures differ from unorganised, which are mostly angular, and of uniform composition externally and internally, by having an external envelope or covering,

See La Description de l'Egypte; ou, Recueil des Observations et des Recherches qui ont été faites en Egypte, pendant l'Expédition de l'Armée Français. Planches. Histoire Naturelle, tom. deuxième, Bis. Botanique, par M. Delile, Planche 1. Palmier Doum.

"Those blazing suns that dart a downward ray,

And fiercely shed intolerable day."-GOLDSMITH. "Palms," says Martius (the distinguished traveller and illustrator of this order of plants)," the splendid offspring of the earth and the sun, chiefly acknowledge as their native land those happy regions where the beams of the latter for ever shine. Inhabitants of either world, they hardly range beyond 35o in the southern, or 409 in the northern hemisphere."

"While some," observes Humboldt, have trunks as slender as the graceful reed, or longer than the longest cable, others are three or even five feet thick; while some grow collected in groups, others singly dart their slender trunks into the air; while some have a low trunk, others exhibit, such as the Ceroxylon andicola, a towering stem 160 to 180 feet high; and while one part flourishes in the low valleys of the tropics, or on the declivities of the lower mountains, to the elevation of 900 feet, another part consists of mountaineers bordering upon the limits of perpetual snows."

which gives them a definite shape, and renders them compact, and which in animals is termed the skin, in plants the bark. The outer portion of plants exposed to the atmosphere exhibits a stratum of cells, colourless and distended with air, having below them one or more strata of cells containing green particles. When the branch or shoot ceases to increase in length, but continues to augment in diameter, the cuticle, made up of colourless cells, frequently splits, either longitudinally, as in the case of the vine, or horizontally, as in the case of the birch. A similar state of things occurs with the subjacent strata which form the epidermis, which either cracks, as in the Scotch fir and cork-tree (Quercus suber), or falls off in large scales or plates, as in the American plane-tree (Platanus occidentalis), the sycamore, and other trees. Below these, even in annual plants, such as the lint (Linum usitatissimum), the hemp (Cannalus sativa), and still more in shrubs and trees, is a stratum of woody fibres, generally of great length, and in certain plants of extreme tenacity, which is the liber or true bark. The fibres of which it consists have a vertical direction in all upright-growing plants, but are intermixed or crossed by cells which have a horizontal position, analogous to the cells proceeding from the pith already spoken of as constituting the medullary rays; one stratum of bark is added to the inner surface as uniformly as a stratum of wood is added to the exterior stratum of wood of the trunk. In some plants the layers of bark cohere very firmly together, as in the cinnamon-tree (Cinnamomum zealanicum); in others they can only be separated by maceration in water; while in others they peel off, or at least are separable by a very slight degree of force, as in various species of tilia or lime-tree, and particularly in the lace-bark-tree of the West Indies (Lagetta lintearia or Daphne lagetta). The outer layers of wood in the stem or branch are in general the only permeable ones, and the protection of this part of a tree is an object of primary importance to the continuation of the living functions of the structure. All injurious influences are warded off by means of the arrangements just detailed-the organisation, the position, and the chemical composition of the bark, all contributing to this end.

The epidermis is well contrived to hinder excessive evaporation, being destitute of pores (or stomata). It likewise prevents the penetration of external moisture, which would occasion the decay or putrefaction of the subjacent tissues. The earthy and often flinty (siliceous) nature of this part helps to ensure this object. Many plants, particularly the equiseta (or horse-tails, Dutch rushes), contain a great quantity of silex, forming a regular coat under the cuticle, so that by slow maceration in water the vegetable matter may be removed, and a tube or perfect cylinder of siliceous crystals remains. The large proportion of silex in the stems of grasses, such as bamboos and the cereal plants, aids also in keeping them erect, notwithstanding the weighty head of grain they have to support.

The bark, moreover, abounds with carbon, or charcoal, which strongly resists the tendency to putrefaction. The bark of the birch has as much as 62, and that of the cork-tree 64 per cent of carbon, which is at least 10 per cent more than any wood yet examined is known to possess. Hence the bark of trees which have been felled, and allowed to lie in humid places, often remains entire after the central part has mouldered away. So thoroughly, indeed, in a growing tree, does the bark prevent the destruction of the sub

• An imitation of the natural protecting covering of plants is attempted to be bestowed on stakes and piles, which are intended to be driven into the earth, by previously charring them. And when the branch of a tree is lopped off, a substitute for the epidermis is had recourse to, in order to preserve the remaining portion of the tree, by covering the cut surface with a sheet of lead, or coating of paint,-expedients far short in efficacy of the original, which has been aptly designated by Sir J. E. Smith as "a fine but essential barrier between life and destruction."