is the principal characteristic by which the action of leaves can be compared to that of animals; the circulation of the fluid being produced by the power of endosmose and contractility on the ascending sap, and that of contractility, gravity, and endosmose on the descending. Digestion appears no part of their action, the absorption of light appearing only an assistant in the much greater chemical action required by plants than animals. The action of plants consisting in the preparation of ternary compounds, as gum, sugar, starch, lignin, &c., and quaternary compounds, as fibrine, albumen, casein, &c., from binary compounds, as water, carbonic acid, ammonia, &c., necessarily implies more chemical action than that of animals, whose food is principally in an organised state, already fit for assimilation, or at least identical in composition with most of the animal tissues, unless the nervous, &c. The decomposition of carbonic acid, water, and ammonia into their elements, and their recombination in a state fit for assimilation by the different organs, require a very great degree of chemical power; and hence full exposure to the direct heat and light of the sun is necessary to plants, to assist the organic action of the leaves in producing these results.

The vital force, Liebig says, is not needed so much in plants as in animals, for the preservation of the tissues from oxygenation; the non-azotised portions of the tissue may be reckoned as comparatively destitute of susceptibility to oxidation, when compared with the azotised portions: hence, he says, the vital force of plants is principally expended in the preparation of new matter, and not wasted as in animals by voluntary and involuntary motions and preservation of tissues; there is therefore more available vital force, and plants are more capable of augmentation in bulk and of forming new matter. He likens vital force, in its developement (not in its character, which he says is distinct), to that of galvanism; as the action of the zinc and acid produces, when in action, a force which may be collected and transmitted along iron rods, so is vital force generated, he says, from oxygenation, and preserved to assist in voluntary and involuntary motion, being transmitted along the nerves to where it is needed. It has not been customary to talk of vital force as a determinate quantity, increasing from the want of motion in one quarter, and being transmitted to another where motion is needed; but the explanation is plausible: the less waste and motion in plants may accumulate vital force, and the almost indefinite capability of extent in plants, as compared with animals, is well known to all practical men, whether it may flow from accumulated vitality or whatever cause. Müller says: "Plants, having only one mode of manifesting life, namely, by vegetation, do not require manifold organs in addition to

their roots, stem, and leaves; and, with the exception of the organs of fructification, transformed from leaves in some cases, present merely a repetition of similar parts, in all of which the simple relation of branches to leaves is the same. A consequence of this is, that each of these parts has the power of becoming in its turn an independent living body; the seed differing from the shoot only in its greater vegetative power." Respiration, he says, affords a very important distinctive character between animals and plants; being performed in plants by the whole surface, and in animals confined to an organ which, in a small space, affords an immense surface for contact with the atmosphere. The difference, however, is more structural than functional: plants give off, or expire, carbonic acid constantly, the same as animals do, though not in so great a quantity; the inhalation of oxygen and exhalation of carbonic acid form a constant function, and seem necessary to their existence. The whole surface, in the dark parts, is said to be capable of this function in some degree, but in the higher classes of plants it is principally confined to the leaves; and the surface of animals is said not to be wholly destitute of this power, even in the more perfect. It is in the inhalation of carbonic acid by the green parts of leaves, and the exhalation of oxygen, that plants differ most from animals. This function is totally different from any thing to be found in animals: it has been called digestion, but seems totally different; and would appear rather a distinct process, necessary to plants only from the greater chemical action required to prepare organised products from inorganic substances. The heat and light of the sunbeam being necessary to perfect the great organic chemical action required, the organs adapted to this function require to be developed externally.

The most remarkable similarity subsists between plants and animals, Müller says, in the process of the developement of their tissues. "The observations of Mirbel had shown that all the forms of vegetable tissue are developed from cells, which at first constitute the whole mass of the tissue, but afterwards undergo various changes in their shape and size, so as to be converted into woody fibre, spiral vessels, &c. M. Schleiden has more recently traced the developement of the vegetable tissue at a still earlier stage. The abundant gum of nascent parts of plants, such as the youngest albumen of a seed, when examined by the microscope, is seen to be turbid from the presence of minute molecules soon larger granules are also observed in it; around these granules, by a kind of coagulation, larger bodies are formed, the cytoblasts, in which the above-mentioned granules are still visible as nuclei. When the cytoblast has attained its full size, a small vesicle appears on it; this enlarges and becomes the cell, in which the cytoblast is for a period still visible, either

attached to its wall or free in its cavity, sometimes permanent. From the observations of animal physiologists, and particularly of Schwann, the process of developement and growth of the tissues of animals are exactly the same. Nearly all the tissues have been shown to be formed from nucleated cells, previously developed in a homogeneous formative mass. The order of developement of the cell and its nucleus or cytoblast, and secondary nuclei within this, as far as it has been observed, appears also to be the same in animals as in plants. Some of the tissues of animals, like the cellular tissue of plants, retain the cellular form, while others, like the more highly developed vegetable tissues, assume different forms." In the growth of marchantia, Mirbel discovered that they increased by the production of new cells alternately between every two of the old; new rows of cells are thus formed, and they extend laterally by additions outside, as well as by superimposed rows. The woody fibres pass downwards from the leaves through the cellular tissue, leaving openings of cellular matter in exogenous plants at the medullary rays, which connect the interior and exterior of the stem. The woody and cellular matter forming the basis of the vegetable structure are thus developed. While the tissue is young and succulent it expands and stretches freely, according to the heat and moisture supplied; and, from the power of endosmose, produced by the light and heat acting from above, thickening the sap by evaporation and chemical action, and attracting the thinner fluid upwards, the expansion is principally in a longitudinal direction upwards, till the leaves are fully formed and commence to solidify and ripen the branch, by the elaborated sap sent downwards. Some plants make the whole of their shoot at once in the early part of the season, others have a spring and autumn growth, and some continue extending the whole season round.

Thus far observation has conduced to establish the theory of developement. We are thus taught that to encourage the soil, the stomach of a plant, to perform its functions, the soil must be properly pulverised in dry weather, as I have often elsewhere repeated in other essays. Too little regard is had to the mechanical operation of the pulverisation of the soil, and taking advantage of tids of weather; and too little allowance is given for unforeseen adverse occurrences of weather and soil, in the experiments instituted in various quarters on manures. To practical men, it is well known that the state of the soil has often more effect on growth than the food deposited; if the stomach of an animal does not perform its functions properly, it will be in vain to load it with food. The function of absorption is increased by keeping the soil loose and porous, to enable the roots to spread and ramify, which is also greatly encouraged by

porous substances in the soil, as pieces of charcoal, bones, decaying wood, porous stones, &c. One of the best materials for encouraging the formation of fibres is rotted leaves, or the charcoal from the half-smothered combustion of the spray of young shoots, leaves, &c. : the leaves and young shoots contain all the substances necessary for keeping vitality active; if the heat is violent it may dissipate the ammonia and fibrine of the shoots, and do harm. The light and heat of the sunbeam are also necessary to produce the phenomena of endosmose, without which absorption and circulation could not go on; electricity also will probably assist in the vital activity or irritability of the tissue, which increases circulation and absorption. To assist aeration, heat and light are necessary to produce exhalation; to promote the interchange of gases, it is necessary that the leaves be kept free of dust and extraneous matter; and when the state of weather out of doors, or confinement in houses, has clogged the leaves, the lungs of plants, it is absolutely necessary to syringe and keep them clean. The important organic chemical action of the leaves, so necessary to prepare the food for assimilation, is also promoted by the same means. Into the functions of the preparation of the food (or digestion), absorption, circulation, and aeration, science has thus enabled us to obtain so much insight as greatly to facilitate and augment their action by proper cultivation. When I come to treat of each of these functions in a special manner, the subject will be rendered more intelligible than it could be in a general outline.

Of the remaining functions of assimilation, secretion, and reproduction, we are more ignorant; of that mysterious power, which from the simple membrane of the organs (to all microscopical and chemical observations seeming alike) can elicit so many and such varied products, we are completely ignorant. Chemical observation of the secreted products, however, and analysis of particular plants, enable us to know what food it is necessary to provide for each; immense additions have lately been, and are still being, made by chemists to our information on these subjects, and great benefit should redound to practice therefrom. Of the same mysterious vital power, which from the rudiments of a branch can prepare such seemingly different products as the parts of the flower, and can change the reproductive bud into the more perfect though more changeable organ the seed, we know also comparatively little. Observation, however, has established that a duly elaborated state of the food is essential; and that where light and heat, or the influence of the sunbeam, cannot be got in sufficient quantity, we can assist the operations of nature by lessening the quantity of sap to be elaborated, and produce the necessary elaborated state for fructification from a small quantity, which our insufficient means will not enable us to do from a larger. larger. By

retarding nutrition we thus increase the tendency to reproduction; and, vice versâ, by picking off flower buds, and increasing food, we increase the tendency to nutritive growth.

Of the manner in which roots, buds, and shoots are produced we know but little in a general way, unless that accumulations of matter, especially vascular, favour their production. It is generally stated by physiologists, that a single cell, in a proper situation and under proper circumstances, is all that is necessary for the preparation of a bud, the nucleus of all growth. Some have thought them connected with the medulla: Mr. Knight thought they were from the alburnum. Masses of buds, however, may be seen in many cases generated in enormous quantities, and crowded together without normal order, from extravasations of sap, on all parts of a plant.

Of that mysterious power which guides the development of the plant, evolving the different organs according to the normal manner of the different species, few have attempted to give any definition. As I noticed before, Müller has likened it to an idea, a picture of the imagination, to which the actions of vitality are constrained to conform, thus causing them to develope after a normal manner, and produce each being after its own kind from the picture. When alterations are made by hybridisation, &c., the picture we can imagine to be conformably altered, and we might thus construct a plausible theory; the great difficulty, however, is to imagine the seat of the sensorium where such picture could be formed. Bonnet and other Continental writers have adopted a different opinion, and contend that all the parts of a plant are contained in embryo in the original germ; and that the actions of vitality only cause a developement of previously formed parts, and not a formation

of new.

Mr. Main, in this country, has been the principal advocate of these opinions. A membrane or indusium, visible or invisible, he says, always surrounds the germ, which contains all the organs of the future plant; all the parts of the plant are afterwards developed from this indusium, in which they are contained, he says, in embryo, and developed as the membrane expands; and it throws off every year a new layer of liber and alburnum, in exogenous plants. As I before noticed, how

ever,

if every thing is contained in embryo previously to being produced, it does not account for accidental interference of hybridisation, or adventitious buds; nor does it allow for leaves and flowers being mutually transformable. It, however, gives a tangible shape to our ideas, by allowing us to conceive of what is invisible, by referring it to something we are already acquainted with; as we conceive of a spirit having bodily organs, though we are only assisting the imagination to comprehend, and are entirely ignorant whether we are right or wrong. If such a thing as an indusium, or germinal membrane