to Knight, then, there are three sets of vessels in leaves, the central tubes, the spiral tubes, and the external tubes. But by what means is the proper juice conducted from the base of the leaf-stalk to the extremity of the root? This was the chief object of the enquiry of the earlier phytologists who had not yet begun to trace its progress in the leaf and leaf-stalk; but who were acquainted with facts indicating at least the descent of a fluid in the trunk. Du Hamel stript sixty trees of their bark in the course of the spring, laying them bare from the upper extremity of the sap and branches to the root; the experiment proved indeed fatal to them, as they all died in the course of three or four years. But many of them had made new productions both of wood and bark from the buds downwards, extending in some cases to the length of a foot; though very few of them had made any new productions from the root upwards. Hence it is that the proper juice not only descends from the extremity of the leaf to the extremity of the root, but generates also in its descent new and additional parts. The experiments of Knight on this subject are, if possible, more convincing than even those of Du Hamel. From the trunks of a number of young crab trees he detached a ring of bark of half an inch in breadth. The sap rose in them, and the portion of the trunk above the ring augmented as in other subjects that were not so treated, while the portion below the ring scarcely augmented at all. The upper lips of the wounds made considerable advances downwards, while the lower lips made scarcely any advances upwards; but if a bud was protruded under the ring, and the shoot arising from it allowed to remain, then the portion of the trunk below that bud began immediately to augment in size, while the portion between the bud and incision remained nearly as before. When two circular incisions were made in the trunk so as to leave a ring of bark between them with a leaf growing from it, the portion above the leaf died, while the portion below the leaf lived; and when the upper part of a branch was stripped of its leaves the bark withered as far as it was stript. Whence it is evident that the sap which has been elaborated in the leaves and converted into proper juice, descends through the channel of the bark, or rather between the bark and alburnum to the extremity of the root, effecting the developement of new and additional parts. But not only is the bark thus ascertained to be the channel of the descent of the proper juice, after entering the trunk; the peculiar vessels through which it immediately passes, have been ascertained also. In the language of Knight they are merely a continuation of the external tubes already noticed, which after quitting the base of the foot-stalk he describes as not only penetrating the inner bark, but descending along with it and conducting the proper juice to the very extremity of the root. In the language of Mirbel they are the large or rather simple tubes so abundant in the bark of woody plants, though not altogether confined to it; and so well adapted by the width of their diameter to afford a passage to the proper juice.

1535. Causes of descent. The proper juice then, or sap elaborated in the leaf, descends by the returning vessels of the leaf-stalk, and by the longitudinal vessels of the inner bark, the large tubes of Mirbel and external tubes of Knight, down to the extremity of the root.

1536. The descent of the proper juice was regarded by the earlier phytologists as resulting from the agency of gravitation, owing 'perhaps more to the readiness with which the conjecture suggests itself than to the satisfaction which it gives. But the insufficiency of this cause was clearly pointed out by Du Hamel, who observed in his experiments with ligatures that the tumor was always formed on the side next to the leaves, even when the branch was bent down, whether by nature or art, so as to point to the earth, in which case the power propelling the proper juice is acting not only in opposition to that of gravitation, but with such force as to overcome it. This is an unanswerable argument; and yet it seems to have been altogether overlooked, or at least undervalued in its importance by Knight, who endeavors to account for the effect by ascribing it to the joint operation of gravitation, capillary attraction, the waving motion of the tree, and the structure of the conducting vessels; but the greatest of these causes is grayitation. Certain it is that gravitation has considerable influence in preventing the descent of the sap in young shoots of trees which have grown upright, which, when bent down after being fully grown, form larger buds, and often blossom instead of leaf buds. This practice, with a view to the production of blossom-buds, is frequently adopted by gardeners (Hort. Trans. i. 237.) in training fruit-trees.-These causes are each perhaps of some efficacy; and yet even when taken altogether they are not adequate to the production of the effect. The greatest stress is laid upon gravitation; but its agency is obviously over-rated, as is evident from the case of the pendent shoots of the weeping willow; and if gravitation is so very efficacious in facilitating the descent of the proper juice, how comes its influence to be suspended in the case of the ascending sap? The action of the silver grain will scarcely be sufficient to overcome it; and if it should be said that the sap ascends through the tubes of the alburnum by means of the agency of the vital principle, why may not the same vital principle conduct also the proper juice through the returning vessels of the bark? In short, if, with Saussure, we admit the existence of a contracting power in the former case sufficient to propel the sap from ring to ring, it will be absolutely necessary to admit it also in the latter. Thus we assign a cause adequate to the production of the effect, and avoid at the same time the transgression of that most fundamental principle of all sound philosophy which forbids us to multiply causes without necessity.

SECT. IV. Process of Vegetable Developement.

1537. The production of the different parts and organs of plants is effected by the assimilation of the proper juice. The next object of our enquiry, therefore, will be that of tracing out the order of the developement of the several parts, together with the peculiar mode of operation adopted by the vital principle. But this mode of operation is not exactly the same in herbaceous and annual plants, as in woody and perennial plants. In the former, the process of developement comprises as it were but one act of the vital principle, the parts being all unfolded in immediate succession and without any perceptible interruption till the plant is complete. In the latter, the process is carried on by gradual and definite stages easily cognisable to the senses, commencing with the approach of spring, and terminating with the approach of winter; during which, the functions of the vital principle seem to be altogether suspended, till it is aroused again into action by the warmth of the succeeding spring. The illustration of the latter, however, involves also that of the former; because the growth of the first year exemplifies at the same time the growth of annuals, while the growth of succeeding years exemplifies whatever is peculiar to perennials.

1538. Elementary organs. If the embryo, on its escape from the seed and conversion into a plant, is taken and minutely inspected, it will be found to consist of a root, plumelet, and incipient stem, which have been developed in consecutive order; and if the plant is taken and dissected at this period of its growth it will be found to be composed

merely of an epidermis enveloping a soft and pulpy substance, that forms the mass of the individual; or it may be furnished also with a central and longitudinal fibre; or with bundles of longitudinal fibres giving tenacity to the whole. These parts have been developed no doubt by means of the agency of the vital principle operating on the proper juice; but what have been the several steps of operation?

1539. No satisfactory explication of this phenomenon has yet been offered. It is likely, however, that the rudiments of all the different parts of the plant do already exist in the embryo in such specific order of arrangement as shall best fit them for future developement, by the intro-susception of new and additional particles. The pellicle constituting the vegetable epidermis has generally been regarded as a membrane essentially distinct from the parts which it covers, and as generated with a view to the discharge of some particular function. Some phytologists, however, have viewed it in a light altogether different, and have regarded it as being merely the effect of accident, and nothing more than a scurt formed on the exterior and pulpy surface of the parenchyma indurated by the action of the air. It is more probably, however, formed by the agency of the vital principle, even while the plant is yet in embryo, for the very purpose of protecting it from injury when it shall have been exposed to the air in the process of vegetation. There are several respects in which an analogy between the animal and vegetable epidermis is sufficiently striking: they are both capable of great expansion in the growth of the subject; they are both easily regenerated when injured (excepting in the case of induration), and seemingly in the same manner; they are both subject, in certain cases, to a constant decay and repair; and they both protect from injury the parts enclosed.

1540. Composite organs. The elucidation of the developement of the composite organs involves the discussion of the two following topics: the formation of the annual plant, and of the original shoot of the perennial; and the formation of the subsequent layers that are annually added to the perennial.

1541. Annuals and annual shoots. If a perennial of a year's growth is taken up in the beginning of winter when the leaves, which are only temporary organs, have fallen, it will be found to consist of a root and trunk, surmounted by one or more buds. The root is the radicle expanded into the form peculiar to the species, but the trunk and buds have been generated in the process of vegetation.

1542. The root or trunk, if taken and cut into two by means of a transverse section, will be found to consist already of bark, wood, and pith. Here then is the termination of the growth of the annual, and of the first stage of the growth of the perennial: how have their several parts or organs been formed?

1543. The pith seems only a modification of the original pulp, and the same hypothesis that accounts for the formation of the one will account also for the formation of the other; but the pith and pulp, or parenchyma, are ultimately converted into organs essentially distinct from one another; though phytologists have been much puzzled to assign to each its respective functions. In the ages in which phytological opinions were formed without enquiry, one of the vulgar errors of the time seems to have been an opinion by which the function of the pith was supposed to be that of generating the stone of fruit, and by which it was thought that a tree deprived of its pith would produce fruit without a stone. (Phys. des Arb. liv. i. chap. 3.) But this opinion is by much too absurd to merit a serious refutation. Another early opinion, exhibiting however indications of legitimate enquiry, is that by which the pith was regarded as being analogous to the heart and brain of animals, as related by Malpighi; who did not. himself adopt it, but believed the pith to be like the cellular tissue, the viscera in which the sap is elaborated for the nourishment of the plant, and for the protrusion of future buds. Magnol thought that it produces the flower and fruit, but not the wood. Du Hamel regarded it as being merely an extension of the pulp or cellular tissue, without being destined to perform any important function in the process of vegetation. But Linnæus was of opinion that it produces even the wood; regarding it not only as the source of vegetable nourishment, but as being also to the vegetable what the brain and spinal marrow are to animals, the source and seat of life. In these opinions there may be something of truth, but they have all the common fault of ascribing to the pith either too little or too much. M. Lindsay of Jamaica suggested a new opinion on the subject, regarding it as being the seat of the irritability of the leaves of the mimosa, and Sir J. E. Smith says he can see nothing to invalidate the arguments on which this opinion is founded. Plenck and Knight regard it as destined by nature to be a reservoir of moisture to supply the leaves when exhausted by excess of perspiration. Hence it appears that the peculiar function of the pith has not yet been altogether satisfactorily ascertained; and the difficulty of ascertaining it has been thought to be increased from the circumstance of its seeming to be only of a temporary use in the process of vegetation, by its disappearing altogether in the aged trunk. But although it is thus only temporary as relative to the body of the trunk, yet it is by no means temporary as relative to the process of vegetation; the central part of the aged trunk being now no longer in a vegetating state, and the pith being always present in one shape or other in the annual plant, or in the new additions that are annually made to perennials. The pith then is essential to vegetation in all its stages: and from the analogy of its structure to that of the pulp or parenchyma, which is known to be an organ of elaboration, as in the leaf, the function of the pith is most probably that of giving some peculiar elaboration to the sap.

1544. The generation of the layer of wood in woody plants, or of the parts analogous to wood in the case of herbaceous plants, has been hitherto but little attended to. If we suppose the rudiments of all the different parts to exist already in the embryo, then we have only to account for their developement by means of the intro-susception and assimilation of sap and proper juice; but if we suppose them to be generated in the course of vegetation, then the difficulty of the case is augmented: and at the best we can only state the result of operations that have been so long continued as to present an effect cognizable to the sense of sight, though the detail of the process is often so very minute as to escape even the nicest observation. All, then, that can be said on the subject, is merely that the tubes, however formed, do, by virtue of the agency of the vital principle operating on the proper juice, always make their appearance at last in a uniform and determinate manner, according to the tribe or species to which the plant belongs, uniting and coalescing so as to form either a circular layer investing the pith, as in woody plants; or a number of divergent layers intersecting the pith, as in some herbaceous plants; or bundles of longitudinal and woody fibre interspersed throughout the pith, as in others. In the same manner we may account for the formation of the layer of bark.

1545. Perennials and their annual layers. If a perennial is taken at the end of the second year and dissected as in the example of the first year, it will be found to have increased in height by the addition of a perpendicular shoot consisting of bark, wood, and pith, as in the shoot of the former year; and in diameter by the addition of a new

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layer of wood and of bark, generated between the wood and bark of the former year, and covering the original cone of wood, like the paper that covers a sugar-loaf: this is the fact of the mode of augmentation about which phytologists have not differed, though they have differed widely with regard to the origin of the additional layer by which the trunk is increased in diameter. Malpighi was of opinion that the new layer of wood is formed from the liber of the former year.

1545. The new layer of wood Linnæus considered as formed from the pith, which is absurd, because the opinion goes to the inversion of the very order in which the layer is formed, the new layer being always exterior to the old one. But according to the most general opinion, the layer was thought to be formed from a substance oozing out of the wood or bark-first, a limpid fluid, then a viscid pulp, and then a thin layer attaching itself to the former; the substance thus exuding from the wood or bark was generally regarded as being merely an extravasated mucilage, which was somehow or other converted into wood and bark: but Du Hamel regarded it as being already an organised substance, consisting of both cellular and tubular tissue, which he designated by the appellation of the cambium, or proper juice.

1547. Knight has thrown the highest degree of elucidation on this, one of the most obscure and intricate processes of the vegetable economy, in having shown that the sap is elaborated, so as to render it fit for the formation of new parts in the leaf only. If a leaf or branch of the vine is grafted even on the fruit-stalk or tendril, the graft will still succeed; but if the upper part of a branch is stripped of its leaves, the bark will wither as far as it is stripped; and if a portion of bark furnished with a leaf is insulated by means of detaching a ring of bark above and below it, the wood of the insulated portion that is above the leaf is not augmented: this shows evidently that the leaf gives the elaboration necessary to the formation of new parts, and that without the agency of the leaf no new part is generated:- Such then is the mode of the augmentation of the plant in the second year of its growth. It extends in width by a new layer of wood and of bark insinuated between the wood and bark of the former year; and in height by the addition of a perpendicular shoot, or of branches, generated as in the shoot of the first year. But if the plant is taken and dissected at the end of the third year, it will be found to have augmented in the same manner; and so also at the end of the succeeding year as long as it shall continue to live; so that the outermost layer of bark, and innermost layer of wood, must have been originally tangent in the first year of the plant's growth; the second layer of bark, and second layer of wood, in the second year; and so on in the order of succession till you come to the layer of the present year, which will in like manner divide into two portions, the outer forming one or more layers of bark, and the inner forming one or more layers of wood. And hence the origin of the concentric layers of wood and of bark of the trunk. But how are we to account for the formation of the divergent layers, which Du Hamel erroneously supposed to proceed from the pith? The true solution of the difficulty has been furnished by Knight, who, in tracing the result of the operation of budding, observed that the wood formed under the bark of the inserted bud unites indeed confusedly with the stock, though still possessing the character and properties of the wood from which it was taken, and exhibiting divergent layers of new formation which originate evidently in the bark, and terminate at the line of union between the graft and stock.

1548. But how is the formation of the wood that now occupies the place of the pith to be accounted for? It appears that the tubes of which the medullary is composed do, in the process of vegetation, deposit a cambium, which forms an interior layer that is afterwards converted into wood for the purpose of filling up the medullary canal.

1549. Conversion of the allmrnum into perfect wood. In consequence of the increase of the trunk by means of the regular and gradual addition of an annual layer, the layers whether of wood or of bark are necessarily of different degrees of solidity in proportion to their age; the inner layer of bark, and the outer layer of wood, being the softest; and the other layers increasing in their degree of solidity till you reach the centre on the one hand, and the circumference on the other, where they are respectively the hardest, forming perfect wood or highly indurated bark, which sloughs or splits into chinks, and falls off in thick crusts, as in the plane-tree, fir, and birch. What length of time, then, is requisite to convert the alburnum into perfect wood, or the liber into indurated bark; and by what means are they so converted? There is no fixed and definite period of time that can be positively assigned as necessary to the complete induration of the wood or bark, though it seems to require a period of a good many years before any particular layer is converted from the state of alburnum to that of perfect wood; and perhaps no layer has received its final degree of induration till such time as the tree has arrived at its full growth. The indura. tion of the alburnum, and its consequent durability, are attributed by many to the loss of sap which the layer sustains after the period of its complete developement; when the supply from the root diminishes, and the waste by evaporation or otherwise is still kept up, inducing a contraction or condensation of its elementary principles that augments the solidity of the layer, in the first degree, and begins the process that future years finish. But Knight believes the induration of the alburnum as distinguishable in the winter to be owing rather to some substance deposited in it in the course of the preceding summer, which he regards as being the proper juice in a concrete or inspissated state, but which is carried off again by the sap as it ascends in the spring.

1550. Circulation of vegetable juices. After the discovery of the circulation of the blood of animals, phytologists, who were fond of tracing analogies between the animal and vegetable kingdoms, began to think that there perhaps existed in plants also a circulation of fluids. The sap was supposed to be elaborated in the root. The vessels in which it was propelled to the summit of the plant were denominated arteries; and the vessels in which it is again returned to the root were denominated veins. Du Hamel, while he admits the ascent of the sap, and descent of the proper juice, each in peculiar and appropriate vessels, does not however admit the doctrine of a circulation; which seems, about the middle of the last century, to have falien into disrepute. For Hales, who contended for an alternate ascent and descent of fluids in the day and night, and in the same vessels, or for a sort of vibratory motion as he also describes it, gave no countenance whatever to the doctrine of a circulation of juices. But the doctrine, as it appears, has been again revived, and has met with the support of some of the most distinguished of modern phytologists. Hedwig is said to have declared himself to be of opinion, that plants have a circulation of fluids similar to that of animals, Corti is said to have discovered a species of circulation in the stem of the chara, but confined, it is believed, within the limits of the internodia. Willdenow has also introduced the subject, and defended the doctrine (Principles of Botany, p. 85.); but only by saying he believes a circulation to exist, and that it is impossible for the leafless tree to resist the cold if there be not a circulation of fluids. Knight has given his reasons somewhat in detail; and

though his doctrine of a circulation should be false, yet the account which he gives of the progress and agency of the sap and proper juice, short of circulation, may be true. The sum of the account is as follows: When the seed is deposited in the ground under proper conditions, moisture is absorbed and modified by the cotyledons, and conducted directly to the radicle, which is by consequence first developed. But the fluid which has been thus conducted to the radicle, mingling no doubt with the fluid which is now also absorbed from the soil, ascends afterwards to the plumelet through the medium of the tubes of the alburnum. The plumelet now expands and gives the due preparation to the ascending sap, returning it also in its elaborated state to the tubes of the bark, through which it again descends to the extremity of the root, forming in its progress new bark and new alburnum; but mixing also, as he thinks, with the alburnum of the former year, where such alburnum exists, and so completing the circulation.

1551. Decomposite organs. To the above brief sketch of the agency of the vital principle in the generation or growth of the elementary and composite organs, there now remains to be added that of the progress and mode of the growth of the decomposite organs, or organs immediately constituting the plant, as finishing the process of the vegetable developement. This will include the phenomena of the ultimate developement of the root, stem, branch, bud, leaf, flower, and fruit.

1552. The root. From the foregoing observations and experiments, it appears that the roots of plants, or at least of woody plants, are augmented in their width by the addition of an annual layer, and in their length by the addition of an annual shoot, bursting from the terminating fibre. But how is the developement of the shoot effected? Is it by the intro-susception of additional particles throughout the whole of its extent; or only by additions deposited at the extremity? In order to ascertain the fact, with regard to the elongation of the root, Du Hamel instituted the following experiment: Having passed several threads of silver transversely through the root of a plant, and noted the distances, he then immersed the root in water. The upper threads retained always their relative and original situation, and the lowest thread which was placed within a few lines of the end was the only one that was carried down. Hence he concluded that the root is elongated merely by the extremity. Knight, who from a similar experiment obtained the same result, deduced from it also the same conclusion. We may regard it then as certain, that the mode of the elongation of the root is such as is here represented, though in the progress of its developement, it may affect a variety of directions. The original direction of the root is generaly perpendicular, in which it descends to a considerable depth if not interrupted by some obstacle. In taking up some young oak-trees that had been planted in a poor soil, Du Hamel found that the root had descended almost four feet, while the height of the trunk was not more than six inches. If the root meets with an obstacle it then takes a horizontal direction, not by the bending of the original shoot, but by the sending out of lateral shoots. The same effect also follows if the extremity of the root is cut off, but not always so, for it is a common thing in nursery-gardens, to cut off the tap-roots of drills of seedling oaks without removing them, by a sharp spade, and these generally push out new tap-roots, though not so strong as the former. When a root ceases of its own accord to elongate, it sends out also lateral fibres which become branches, and are always the more vigorous the nearer they are to the trunk, but the lateral branches of horizontal roots are the less vigorous the nearer they are to the end next the trunk. In the former case, the increased luxuriance is perhaps owing to the easy access of oxygen in the upper divisions; but in the latter case, the increased luxuriance of the more distant divisions is not so easily accounted for, if it is not to be attributed to the more ample supply of nutriment which the fibres meet with as they recede from the trunk, particularly if you suppose a number of them lying horizontally and diverging like the radii of a circle. But the direction of roots is so liable to be affected by accidental causes, that there is often but little uniformity even in roots of the same species. If plants were to be sown in a soil of the same density throughout, perhaps there might be at least as much uniformity in the figure and direction of their roots, as of their branches; but this will seldom happen. For if the root is injured by the attacks of insects, or interrupted by stones, or earth of too dense a quality, it then sends out lateral branches, as in the above cases; sometimes extending also in length by following the direction of the obstacle, and sometimes ceasing to elongate, and forming a knot at the extremity. But where the soil has been loosened by digging or otherwise, the root generally extends itself to an unusual length, and where it is both loosened and enfiched, it divides into a multiplicity of fibres. This is also the case with the roots of plants vegetating in pots, near a river, but especially in water. Where roots have some considerable obstacle to overcome they will often acquire a strength proportioned to the difficulty: sometimes they will penetrate through the hardest soil to get at a soil more nutritive, and sometimes they will insinuate their fibres into the crevices even of walls and rocks which they will burst or overturn. This of course requires much time, and does much injury to the plant. Roots consequently thrive best in a soil that is neither too loose nor too dense; but as the nourishment which the root absorbs is chiefly taken up by the extremity, so the soil is often more exhausted at some distance from the trunk than immediately around it. Du Hamel regards the small fibres of the root which absorb the moisture of the soil as being analogous to the lacteals of the animal system, which absorb the food digested by the stomach. But the root is rather to be regarded as the mouth of the plant, selecting what is useful to nourishment and rejecting what is yet in a crude and indigestible state; the larger portions of it serving also to fix the plant in the soil and to convey to the trunk the nourishment absorbed by the smaller fibres, which ascending by the tubes of the alburnum, is thus conveyed to the leaves, the digestive organs of plants. Du Hamel thinks that the roots of plants are furnished with pre-organised germs by which they are enabled to send out lateral branches when cut, though the existence of such germs is not proved; and affirms that the extremities of the fibres of the root die annually like the leaves of the trunk and branches, and are again annually renewed; which last peculiarity Professor Wildenow affirms also to be the fact, but without adducing any evidence by which it appears to be satisfactorily substantiated. On the contrary, Knight, who has also made some observations on this subject, says, it does not appear that the terminating fibres of the roots of woody plants die annually, though those of bulbous roots are found to do so. But the fibres of creeping plants, as the common crowfoot and strawberry, certainly die annually, as do those of the vine.

1553 The stem. The stem, like the root, or at least the stem of woody plants, is also augmented in width by the addition of an annual layer, and in length by the addition of an annual shoot bursting from the terminating bud. Is the developeinent of the shoot issuing from the stem effected in the same manher also? The developement of the shoot from the stem is not effected in the same manner as that of the root by additions to the extremity only, but by the intro-susception of additional particles throughout its whole extent, at least in its soft and succulent state: the longitudinal extension diminishing in proportion as the shoot requires solidity, and ceasing entirely when the wood is perfectly formed; though often continuing at the summit after it had ceased at the base. The extension of the shoot is inversely at its induration, rapid while it remains herbaceous, but slow in proportion as it is converted into wood. moisture and shade are the most favorable to its elongation, because they prevent or retard its induration;

Hence

and hence the small cone of wood which is formed during the first year of the plant's growth increases no more after the approach of winter, neither in height nor thickness. Such is the mode of the growth and developement of the trunk of perennial and woody plants, to which there exists a striking exception in the growth of the trunk of palms. Their internal structure has been already taken notice of as presenting no concentric or divergent layers, and no medullary canal, but merely an assemblage of large and woody fibres, interspersed without order in a pulp or parenchyma, softer at the centre and gradually becoming harder as it approaches the circumference. When the seed of the palm-tree germinates, it protrudes a circular row of leaves, or of fronds, which crowns the radicle, and is succeeded in the following year by a similar row issuing from the centre or bosom of the former leaves, which ultimately die down to the base. This process is continued for four or five years successively without exhibiting as yet any appearance of a stem, the remaining bases of the leaves or frond forming by their union merely a sort of knob or bulb. last, however, they constitute by their union an incipient stem, as thick the first year as it ever is after; which in the following year is augmented in height as before, and so on in succession as long as the plant lives, the leaves always issuing from the summit and crowning the stem, which is a regular column, but decaying at the end of the year, and leaving circular marks at the points of insertion, which furrow the surface of the plant, and indicate the years of its growth.

At

1554. The branches, in their mode of growth and developement, exhibit nearly the same appearances as the trunk from which they issue. They originate in a bud, and form also a cone that consists of pith, wood, and bark; or rather they form a double cone. For the insertion of the branch into the trunk resembles also a cone whose base is at the circumference, and whose apex is at the centre, at least if it is formed in the first year of the plant's growth, or on the shoot of the present year; but falling short of the centre in proportion to the lateness of its formation, and number of intervening layers. Branches in their developement assume almost all varieties of position from the reflected to the horizontal and upright; but the lower branches of trees are said to be generally parallel to the surface of the soil on which they grow, even though that surface should be the sloping side of a hill-owing, as it has been thought, to the evolution of a greater number of buds on the side that forms the obtuse angle with the soil, in consequence of its being exposed to the action of a greater mass of air.

1555. The bud, which in the beginning of spring is so very conspicuous on the trees of this country as to be obvious to the most careless observer, is by no means common to all plants, nor to plants of all climates; shrubs in general, and annuals universally, are destitute of buds as well as all plants whatever growing within the tropics, the leaf being in them immediately protruded from the bark. It is only in the woody plants of cold climates, therefore, that we are to look for buds; and in them no new part is added, whether proper to the leaf or flower, without the intervention of a bud. For when the young shoot is produced, it is at the same time furnished with new buds, which are again extended into new shoots in the following spring; and thus the bud is to be regarded as forming, not only the cradle but also the winter quarters of the shoot, for which its coat of tiled and glutinous scales seems admirably well adapted. It is found chiefly in the extremity, or on the surface of the young shoot or branch, and but rarely on the stem, except it be at the collar where it produces suckers. It is also generated for the most part in the axil of the leaves, as may be seen by inspecting the annual shoot of almost any tree at random, though not universally so; for to this rule there exists a curious and singular exception in the bud of the platanus, which is generated in the very centre of the base of the foot-stalk, and is not discoverable till after the fall of the leaf. But how are the buds formed which are thus developed? Malpighi thought they were formed from the pith or cellular tissue, which the latter regarded as viscera destined for the elaboration of the sap and protrusion of future buds. Du Hamel thinks the exterior scales of the bud originate in the interior part of the bark, and Knight relates an experiment from which he thinks it follows that the buds are formed from the descending proper juice. But whatever may be the actual origin of the bud, it is evident that its developement does not take place except through the medium of the proper juice, which has been elaborated in the leaves of preceding buds, and originally in those of the plumelet; as the young bud does not make its appearance till the leaves of the preceding buds have expanded, and will not ultimately succeed if deprived of them too soon.

1556. Bulbs are so very similar to buds both in their origin and developement as to require no specific investigation.

1557. The leaf. When the leaves burst from the expanding bud, and even long before that period, as may be seen by the dissection of the bud in the winter, they are complete in all their parts. Hence it is obvious that the leaf, like the young shoot, effects its final developement by means of the intro-susception of new particles throughout the whole of its dimensions: and yet this law of developement is not common to all leaves whatever, for the leaves of liliaceous plants extend chiefly at the point of their junction with the bulb. The effect perhaps of their peculiarity of structure, in being formed' of parallel tubes which extend throughout their whole length, without those transverse and branching fibres that constitute what are called the nerves of the leaves of woody plants.

1558. The flower and fruit. When the flower bursts from the expanding bud, and even long before that period, it is already complete in all its parts, as may be seen also by the dissection of the bud in winter. Linnæus represents the pistil as originating in the pith, the stamens in the wood, and the corolla and calyx in the inner and outer bark respectively: but this account of their origin, though extremely plausible at first sight, will not bear the test of minute examination, being contradicted by the anatomy of the parts themselves; particularly in the case of compound flowers. Knight in investigating the organisation of the apple and pear, endeavored to ascertain the origin of the several parts by tracing the organs of the fruit-stalk to their termination. In the fruit-stalk he thought he could discover the pith, the central tubes, spiral tubes, and tubes of the bark, together with its epidermis: and in tracing them to their termination, he thought the pith seemed to end in the pistils; the central vessels in the stamens, after diverging round the core and approaching again in the eye of the fruit; and the bark and epidermis in the two external skins. Hence he infers that the flower is a prolongation of the pith, wood, and bark. A question of some considerable importance has arisen out of this subject: does the flower or fruit elaborate sap for its own developement, or is it supplied with nourishment from the leaf? By placing small branches of the apple, pear, and vine, with blossoms not expanded, in a decoction of logwood, Knight found that the central vessels were colored by the decoction. By means of a similar experiment on the same subjects after the fruit was formed, the coloring matter was traced through the mass of the fruit to the base of the stamina. And hence it appears that the flower and fruit do possess the power of elaborating sap for their own developement. Knight infers from the foregoing data, that the blos som is nourished from the alburnum, by means of the mingling of the proper juice, which the alburnum may be supposed to contain with the sap in its ascent.

SECT. V. Anomalies of Vegetable Developement.

1559. A deviation from the general laws of developement is occasioned by the intervention of some accidental cause; or of some cause operating permanently in certain subjects. Hence the anomaly may regard the developement either of an individual or a species, and may occur either in the root, stem, branch, leaf, bud, flower, or fruit, according to the circumstances in which it is placed; or it may effect the habit, duration, or physical virtues of the plant.