vesicles, as they may be termed, may possibly also be compounded in like manner, and so on to an extent which it is utterly beyond our powers to comprehend. To the limits of minute division in the works of the great Creator there appears no bound. Were it not that all parts of his creation clearly prove that magnitude and number present no restraint to his operations-that the vast and the minute are equally the object of his attention-we should often be tempted to throw aside many of the sublime truths of natural science as little better than the wild reveries of a heated imagination. But every department of scientific research leads to the same conclusions, overwhelming as they are to the powers of the human intellect, and every mode of investigation teaches the same results; and while we make the vain attempt to conceive the wonders of creation, we can only pause and admire when the faint glimmering perception, which alone we are able to obtain, arises in our minds.

But to proceed, without entering further into a question which is quite beyond our powers-this green powdery crust in its elementary composition-elementary, as it appears to our eyes, may represent what is to us the ultimate forms of vegetable organization. A simple vesicle connected in this instance with similar vesicles by a quaternary arrangement, in others, as in some of the Confervæ and various plants of the Algæ tribes, by a binary, ternary, or linear mode of arrangement.

When the connection between the organic particles is more close, the vesicles being as it were brought within the sphere of mutual attraction, we have them not merely in contact, but variously compounded-two together as in the granules of Gracilaria erecta-three together as in the ternate granules of Rhodomela pinastroides-or lastly, four or more as in the compound granules of Microcladia glandulosa.

If a number of these elementary vesicles be so arranged in simple contact as to take up the smallest possible space, a section of them will present the appearance of a series of circles, or if subjected to a certain degree of equable pressure, they will form a kind of network, pentagonal, hexagonal, or otherwise, according to the presence of various modifying circumstances.

In thus regarding the arrangement of the ultimate vesicles of the vegetable organization to form the cellular structure of plants, we must not confine our attention to the mere mechanical principles of juxta-position and pressure. Each organic vesicle, in a state of activity, is endowed, either in itself, or as appertaining to the vegetable structure of which it forms a part, with the principle of life. It may, therefore, increase in substance and expand in size; and from this gradual expansion or growth, certain changes in its development and in its apparent structure will necessarily take place. Thus, by the gradual expansion of a simple vesicle, it will attain to a larger size--its shape may

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become elongated or depressed, according to the circumstances of pressure under which the expansion takes place-the outer filmy covering or envelope may increase in thickness, and the secondary vesicles of which this film consists become so far developed as to be sensible to our modes of investigation—and lastly, the organic molecule may acquire the property of reproduction, and by the formation of other vesicles from the inner surface of its filmy covering, or more probably by the development of the pre-existing particles of which this covering is composed, the vesicle which was either simple, or to our senses apparently so, becomes compound, having its central portion occupied with other and secondary vesicles. That these are not merely speculations has been fully established by the researches of M. Raspail upon the minute chemistry of organic products. In examining the granules or vesicles of fecula, a vegetable product of great importance, and of which the seeds and roots of many plants almost entirely consist, he observed that, subjected to a limited artificial heat, the membrane or filmy envelope constituting their external coating was capable of gradual extension, the granules increasing in size, and that at the same time globules were formed all over its surface, resembling new grains of fecula, attached by a hilum or minute scar to a membrane. Analogy," observes M. Raspail, seems to point out before hand that, under the influence of the natural causes which affect vegetation, whose action though slow is durable, this development should possess a more regular character than under the influence of an artificial cause. What is thus indicated by analogy is proved by observation, in regard to the grains of fecula, which grow in the vegetable organs, till their forms and dimensions might render it difficult to recognize them. On the other hand, direct observation shows that, in consequence of this development, there are formed in the interior of each grain of fecula, new feculent globules, which being packed together, present a very perfect resemblance to the cellular texture. Each of these grains of fecula, when obtained separate, is furnished with a hilum, by which it was connected with the inside of the cell which contained it, just as a bean is connected by its hilum with the placental parietes of the large cell which we call the pod. But this bean is not merely stuck on to the surface of the pod; it has been developed by the progressive swelling of the inside, and has passed successively through every dimension from that of a microscopic globule to that which it possesses when ripe."-"The secondary grains, whose presence we have detected in its interior (i. e. of the primary vesicle), acted on by the same causes, will be developed in their turn, and will in turn give rise to other tertiary grains, and so on indefinitely; so that we shall then have a more or less complicated cellular texture in the interior of a single cell. Now as these new cells are, at every period of their growth, attached to the

sides of the generating cell, we can conceive that each of them is nothing else than the development of one of the globules of which the coats of the cell are composed."*

It has been before observed that the development of the cells of plants is materially influenced by the varying circumstances of pressure under which it takes place. When the pressure is unequally distributed, being less in one direction and at the same time considerable in all others, the cells will be much modified in their shape. They will be compressed according to the direction in which the pressure is applied, and the active principle of growth continuing to be exerted chiefly in the direction of least pressure, a series of elongated cells will be formed, approaching more or less to the form of tubes. To this modification of cellular structure, M. Link has given the name of elongated tissue. Sometimes the elongated cells, instead of being cylindrical or prismatic in their shape, are fusiform or spindle-shaped, that is, narrower at the extremities than in the centre. These are the clostres of M. Dutrochet. The medullary rays or striæ found in the stems of dicotyledonous plants are also elongated cells, but placed in a horizontal position, instead of a vertical one, which is the usual arrangement of the cellular texture.

Such is the elementary structure and development of the cellular tissue of plants; we have now to direct our attention to the various modifications of vessels constituting the vascular tissue. 66 Vessels," says M. Richard, "are layers of elementary cellular tissue, rolled up in such a way as to form canals, or cells, which are more or less elongated and placed end to end, and whose partitions have often disappeared."+ This definition of vessels, though given by one of the latest and best authorities, is extremely vague and imperfect, and affords us little insight into the gradual or progressive development of this part of the vegetable organization. From what has been stated before respecting the modifications of the cellular configuration induced by pressure, it will appear upon consideration that there is a tendency, under certain circumstances of pressure, to a transition from the cell to the vessel or tube: the elongated cell, for instance, is in fact a short tube, closed at both ends, and it is precisely under the circumstances of pressure by which the elongated cell is produced that we find the vascular structure to be developed. The lateral pressure in vascular plants, arising from their external form, is very considerable; whereas the pressure in the cellular plants is for the most part much more equally distributed. It does not seem improbable, also, that the mode in which the cells are arranged together may exert some influence upon the development of the vascular structure. When arranged in close contact, there will

*Raspail. Elements of Organic Chemistry, translated by Henderson, p. 2. Richard's Elements of Botany, by Clinton. Ed. 4. p. 15.

be a greater tendency to form the hexagonal cellular tissue, but if placed in a linear arrangement, and subjected at the same time to lateral pressure, the cells will tend to become elongated, and not yielding laterally, the filmy envelopes will give way at their points of contact, and a communication be thus formed from one cell to another, constituting at length a continuous tube. Let us now examine the several varieties of vessels with the view of ascertaining how far the opinions here advanced are in accordance with the more evident phenomena presented by the vegetable organization. At the points of union between the stem and the root, and the stem and the branches, are found vessels of a moniliform or beaded configuration. They are apparently jointed, and, according to some botanists, are traversed interiorly by transverse partitions. Their external coats, as well as their internal partitions, are formed of areolar cellular tissue; that is, of cellular tissue in which (probably from the secondary cells of which it is composed not approximating very closely) interstitial spaces are left. They, therefore, present the appearance of being perforated. The slightest inspection is sufficient to prove that these beaded vessels are nothing more than cells arranged in a linear series, and which, in consequence of the interstitial character of their coats, have become united together, so as to form an imperfect tube or vessel, the internal partitions remaining in some instances, and disappearing in others, according to the progress made in the development of the true vascular structure.

In the woody layers of the root, stem, and branches, vessels nearly allied to the preceding are found to occur. These are the punctuated vessels of some botanists, the porous vessels of M. Mirbel. They appear to be continuous tubes, having their sides perforated by pores, which are arranged in transverse lines around the tube. We may readily conceive the beaded or jointed vessel when subjected to considerable lateral pressure, such as must take place in the parts of the plant where these punctuated tubes are found losing the jointed appearance, and having its internal partitions removed. When the pores which are found in the sides of these vessels run into each other, forming apparent transverse slits, the tubes are called false spirals, to distinguish them from the true spiral vessels. The formation of these last, the true spirals, appears to be somewhat different from the preceding, and to arise from the development of the secondary vesicles constituting the envelope of what may be termed, the primary cells. M. Raspail is of opinion that these secondary vesicles are so arranged in the filmy tissue of the envelope of the primary vesicle as to pass round the cell in a spiral direction, instead of in parallel rows or rings. Now if a cell of this description be gradually elongated, and increase in its progressive development according to the principles before laid down, a tube or vessel presenting precisely the characters of the spiral will result. The whole primary cell is developed and elongated

so as to form a cylindrical tube of greater or less extent, its central cavity constitutes the internal tubular cavity of the vessel, and the secondary vesicles of its envelope still retaining their spiral arrangement, form the spiral external coating of the vessel. The communication between several of these elongated cells or tubes may afterwards take place, so as to form a continuous vessel of considerable extent in the same manner as in the more

simple vessels. These spiral vessels are very readily seen in the leaves of many plants. By carefully breaking a strongly nerved leaf, and gently drawing the parts asunder, the spiral vessels of the nerve will become visible, and the spiral texture may be unrolled to a considerable extent without breaking.

These several kinds of vessels pass into each other, forming the mixed vessels of Mirbel; the same vessel being in one part of its extent porous, in another transversely slit, and in a third spiral. The explanation of this modification of vascular development appears to be that cells and tubes formed under varying circumstances of pressure or other modifying causes, and consequently differing in the arrangement of the particles of which they consist, have become united together so as to form a continuous vessel.

There are two other kinds of vessels which require to be noticed. These are simple tubes and the proper vessels. The proper vessels of plants are short tubes without pores, and are found in the bark, leaves, and flowers. They contain the peculiar secretions of the plants in which they are found, as the yellow acrid juice of the Celandine Cheledonium majus, the opaque milky juice of the poppies, &c. They are, therefore, elongated cells, appropriated as reservoirs for the proper juices furnished by the individual plants. The simple tubes are vessels which are without pores, and appropriated to the circulation of the sap, and hence called sap-vessels. They do not differ in their mode of development from other vessels, but in consequence of the secondary cells of their coats being more closely compacted, no interstitial spaces or pores are visible in their sides.

In the more simple and elementary forms under which plants present themselves to our notice, the cellular texture alone is found to occur. These plants, the Cellulares, or Cellular plants of De Candolle, constitute one of the primary divisions of the vegetable kingdom. They differ greatly, both in their internal structure and external configuration, in the several organs and parts of which the vegetable frame consists-in their roots, stems, branches, leaves, and parts of fructification, not only from plants of a higher order, but also among themselves. There is little apparent analogy between the rose which delights us with its fragrance, and the noisome fungus, Nature's scavenger-between the sturdy oak, and the crust which attaches itself to the surface of the solid rock, either as a whole or in any of their parts; and yet the transition from one of these extremes to the other, through