Although, therefore, the filices, in the organs of their embryo, possess a certain degree of analogy with dicoty ledonous plants, this analogy diminishes as the process of germination advances. On cutting across the stem of dicotyledonous trees at the end of the first summer, three distinct parts are manifest ; the bark enveloping the whole, a thin circle of wood immediately under it, and in the cen tre a great proportion of spongy cellular matter. 'rho young stein thus formed resembles a cone, whose summit is terminated by a bud. During the succeeding seasons, the stem preserves its original shape, only the summit of the cone is extended, and its diameter enlarged ; and on cutting it across now, we find the proportion of wood great ly increased, and that of the central spongy matter dimi nished, while it is horizontally radiated through the wood towards the circumference of the section. The origin of branches in these trees, is effected precisely in a similar manner. A lateral bud is evolved from the bark, whose centre is produced from the radiated cellular matter of the trunk, surrounded with spiral sap•vessels; and a cone of wood is in like manner formed around it, whose base, du ring the first season, is gradually encircled by a layer of the contemporaneous.maternal wood.
During the second year, therefore, the base of the young shoot can receive no addition to its proper diame ter, except where it is unencumbered by the wood of the trunk : so that at its origin, a branch resembles the summit of an inverted cone, enveloped by the circles of maternal wood.
As to the order of the developement of the various parts of the stem,,,Alalpighi supposed, that the internal cir cles of bark were subsequently condensed into wood; while Grew concluded, that the young wood was elaborated by the bark.
But the well-devised experiment of Duhamel demon strated, that the•bark itself, as well as the young wood, de rive their origin from the gelatinous part of the sap, (the cambium,) exuding from the central and radiated cellular substance, in which new sap-vessels are developed annual ly during the season. The experiment of Duhamel al luded to, consisted in removing completely the' bark of a cherry-tree, front the trunk of which lie afterwards ob served the sap oozing, and forming a new bark, under which new circles of wood were afterwards formed.
We are aware, that the accuracy of this experiment has been lately called in question by Palisot de Beauvois, in a memoir read before the Institute of France,who observes, that when a portion of the bark of a tree is removed, and the part from which it is taken is well rubbed, so as to leave no remains either of bark or cambium, neither the young nor old wood produce any thing, but that the edges of the divided bark extending over the bare wood then produce new wood, which unquestionably is derived from the former bark.
But it does not seem that this experiment of M. de Beauvois invalidates the truth of M. Duhamel's conclu sion ; on the contrary, it is perfectly reconcilable with it. The efforts of nature by no means limited to one mode of effecting her purpose. In fact, in both experiments the bark was formed from the cambium, from which the bark first, and subsequently the new wood, derive their origin; but in De Beauvois's experiment, the cambium oozed from the trunk covered with the remaining bark ; whereas in Duhamel's, it necessarily exuded from the bare trunk.
It ought to be mentioned to the credit of Malpighi, that Duhamel's experiment partly confirmed the sagacity of his opinion, ,‘ Conquitur itaque in horizontalibus utriculis et medulla ipsa succus, ut futuris et proxime erupturis gem mis, et tenellis foliis Prxsto sit."* For it is evident that the increased diameter and elongation of these plants de pend on the same cause, viz. the annual expansive mo tion of the sap in the lymphatic vessels and cellular sub stance of the stem, and the subsequent formation and con densation of both into bark and wood, assimilated proba bly by the proper juices of the respective plants, prepared by the leaves, and distributed through the descending ves sels.
From this general view of the manner of growth and organization of the stems of dicotyledonous trees, we shall perceive how t emarkably they differ in both from ferns.
The second period of the growth of these plants may be said to commence, when the seminal lobes, having attain ed their full size, a circular opening appears at the point of their union, in the centre of which the first frond may be perceived in that state of involution which is common to ferns with other kindred tribes. Even at this early pe riod, the frond is at once distinguished from the seminal lobes. It is of a paler green colour, and with the stipes of a triangular form, in several species resembling the let= ter r, (Plate CCLIV. Figs. I, 2, 3, 4, 5, 6.) and, with the microscope, vessels may be seen elegantly ramified, di verging from the central stipes in every direction; where as, in the seminal lobes, which are cellular and of a darker green hue, no ramified vessels can be distinguished. The temporary fibrous radicles of the seminal lobes, as well as themselves, now no longer of use, begin to fade, and theit dark green sap being evidently absorbed for the nutrition of the young plant, from the tubercus stem of which the true root now descends exactly as in dicotyledonous plants. A second frond is soon perceived shooting from the exit of the first, and opposite to it, but with an additional lobe. In the same manner the fronds are thus evolved one after another, with a gradual addition to the number of lobes ; the stem, during this period, acquiring only a slight addi tion of diameter, without any perceptible increase of length. Another circumstance to be remarked in the economy of these plants, is, that even during the second year, the stems, at• least of the native European species, frequently germi nating in the crevices of our sandstone cliffs and Gothic ruins, are very far from attaining their utmost diameter. It is no doubt probable, that the slow growth of these plants must be greatly influenced by their situation and soil, as well as by the temperature of our northern cli mate, but ill adapted to foster the growth of plants, which only attain their full perfection in the luxuriant forests of the tropics. Were it, however, admissible, under cir cumstances where observations are greatly deficient, to derive the grounds of probability from analogy, it is pro bable, that even under circumstances the most favoura ble, ferns do not attain their utmost diameter during seve ral successive seasons; and it would seem that these plants possess a singular analogy with palms, in this and other respects.