coincides precisely with the result. There cannot be a more complete proof of the absence of any specific antagonism or vital force than is presented in these and similar experiments; the gases expired are distinctly the same as should flow out by exosmosis. 34. The property of evolving nitrogen in the families of fungoid Cryptogamia, associates them to the Vasculares, and shows that whatever points of difference may exist between these divisions of the vegetable kingdom in other respects, there is in this respect an uniformity of action of the greatest interest, as the chemical changes leading to the separation of nitrogen, belong to all living plants. The large absorption of oxygen in fungi is a prominent function, they do not appropriate it in the same limited way as other plants, but are even capable of decomposing water for its attainment, the hydrogen being liberated. This phenomenon has been witnessed by Humboldt, De Candolle, and M. Marcet. 35. In the researches of Th. de Saussure on germination, an interesting case of physical penetration occurs, which has produced some confusion among theorists. In the Memoires de la Societé Physique, &c., de Genève, t. vi, p. 545, it is stated that seeds germinating in common air absorb nitrogen gas; but that when the process is conducted in an atmosphere of equal volumes of nitrogen and oxygen, none is absorbed. This is precisely the result to be expected, on the hypothesis that the absorption of gases is a physical phenomenon: so long as the atmosphere without contains 80 per cent. nitrogen, this gas is absorbed to produce a compensation in the plant gas, but when it is reduced to 50 per cent. absorption ceases, the seeds and growing parts, containing a sufficiency in their pores. 36. How far confusion may arise in questions of vegetable physiology, if we overlook the physical laws of penetration, is made evident by the contradictory statements of Saussure, Ingenhousz, Plenk and De Candolle, on the absorption and evolution of gases by the green parts of plants placed in artificial atmospheres. Les parties verts laissent moins de gas oxigène dans le gas hydrogène que dans le gas azote; elles ne paraissent, contre l'assertion d'Ingenhousz, absorber ni l'un ni l'autre. Il parait aussi certain, malgré l'assertion d'Plenk, qu'elles n'exhalent point de gas azote, sauf dans quelques cas, par les corolles.-De Candolle Physiologie Veg., t. i, p. 133. 37. Some observations made by me in the summer of 1844, also serve to verify the position, that the gases exhaled by plants ⚫ are not constant, but depend upon the atmosphere in which they are plunged. Specimens of the Conferva mucosa were placed in pump-water, which is their natural medium, and others in distilled water impregnated with carbonic acid gas, and exposed to sunlight. In six hours the gases produced in the receivers were entirely separated, and the arrangement left for twenty-four hours without any fresh water; the gases were again withdrawn, and thus for four and five days, no fresh water being added throughout. It is evident that the gas of the water was constantly changing in its composition, and therefore the experiments were the same as if made in a number of different artificial mixtures of gases. The plants in pump-water gave in the first six hours an expired gas, consisting of oxygen 73, nitrogen 27 per cent.; in twentyfour hours, oxygen 53, nitrogen 47 per cent.; in forty-eight hours, oxygen 18-6, nitrogen 81-4 per cent. The specimens in carbonated water produced in six hours, gas consisting of oxygen 68, nitrogen 32 per cent.; in twenty-four hours, oxygen 63, nitrogen 37 per cent.; in forty-eight hours, oxygen 12, nitrogen 88 per cent.; in seventy-two hours, oxygen 3.5, nitrogen 96.5 per cent. These latter were in no way injured at this time, for upon adding a little fresh fluid they yielded at ninety-six hours, a gas consisting of oxygen 15, nitrogen 85 per cent. The aliment of these plants was not changed, for they continued healthy, the different gases expired were merely the result of physical necessity, the aëriform matter of the water being continually changed. I do not assert that all the nitrogen in the foregoing measures was thrown out from the interior of the plants, because as we have already shown in section 24, the physical disturbance of the water gas will be attended with the evolution of nitrogen, and although distilled water was used, there is no process by which all its gas can be driven out. 38. Conclusion.-From the preceding evidence I infer that plants are a simple porous system, so far as they are related to the air and the gases of fluids in the soil, leaving out of consideration the internal phenomena of penetration. The advantages resulting from the adoption of this philosophical view of vegetation, both in assimilating facts hitherto insulated and criticising experimental arrangements in vegetable physiology, form its great recommendation. For illustration, we adduce two general laws which spring from this theory. 1st. No hypotheses nor arguments can be based on the composition of the gases expired by plants, without a rigorous regard to the influence of disturbing causes-as the amount of light, gas of the fluids of the soil, of the atmosphere, &c. 2d. No experiments on the action of plants in sunlight or otherwise, can be adduced for physiological argumentation, unless made in atmospheric air. Many observations have been made on plants immersed in water and artificial atmospheres, which cannot be received, because the gases employed have penetrated the interior in proportions differing from those in the case of common air. The nitrogen obtained by Saussure may in a great measure have been derived from the additions of carbonic acid made by him to the atmospheres in which he experimented; and in the observations made in water, much of the nitrogen is unquestionably derived from the fluid. 39. Finally I would present the following summary of conclusions as fairly deduced from the preceding experiments. 1. The epidermis of plants, so far as experiments have been made, is porous and permits the passage of gases, according to the physical laws of penetration. 2. The roots, during the existence of chemical changes in plants, absorb such gases only from the soil fluids with which they are in contact, as will indirectly satisfy the indications of the internal atmosphere. 3. The internal gas of plants, or plant atmosphere, is continually fluctuating with the forces which operate upon it; during a state of activity in the plant, it resembles a mixture of nitrogen 86-75, oxygen 13-25 per cent., but at night appears to contain more oxygen, and from 2 to 3 per cent. of carbonic acid. 4. Its active or normal composition is that indicated by a mixture into which carbonic acid and oxygen are being diffused during day-light. 5. The porosity of the entire plant is fully established by its action on artificial atmospheres. Therefore the physical structure of plants is that of a porous system, subject to all the laws of diffusion, and endowed with no vitality other than that resulting in the formation and development of Cytoblasts and their arrangement after a definite type. New York, March, 1846. ART. VII.—On Zoophytes; by J. D. DANA.* THE singular features of the growing coral field, the resemblance to vegetation in its productions, as well as their beauty and variety, have long excited the attention even of those little curious in the forms of living nature. Trees, shrubs, and other plants of various kinds are represented with wonderful exactness, as if they had been the types of this branch of the animal kingdom; and they grow mingled together often in rich profusion like the plants of the land. The similarity, moreover, is not confined to general form: corals have their blossoms; for polyps are flowers both in figure and beauty of coloring. Like the pink or Aster, they have a star-like disk above; and while some are minute, others are half an inch or even two inches in diameter. Every part of a Madrepore when alive is covered with these blossoms: a Gorgonia, though merely a cluster of naked stems, as seen in our cabinets, consists, when in the water, of as many crowded spikelets of flowers. Thus it is with all zoophytes. Nothing could be more untrue than the night-mare dreams of a favorite poet.† "Shapeless they seem'd, but endless shapes assumed; And again, they are described as issuing from the coral, like "capillary swarms Of reptiles horrent as Medusa's snakes." Polyps are not writhing worms. The choicest garden does not produce flowers of more graceful figure or gayer colors, than those of the zoophyte reef; and we may add too, that the birds of the groves will not rival the rich tints of the fishes that sport * In the series of articles on zoophytes, which it is proposed to prepare for this Journal, the writer presents the facts and principles that have been published in his Report on Zoophytes, one of the volumes of the late Exploring Expedition under Capt. Charles Wilkes, (see this Journal, Second Series, vol. i, p. 178.) The subject is however condensed, and the stile and arrangement altered to adapt it to these pages, and give it a somewhat more popular character. It is the writer's endeavor to present a succinct account of this department, about which there is little generally known, without confining himself to original observations. † Montgomery's Pelican Island. among the coral branches. The coral tree is without verdure, but there is full compensation in its perpetual bloom. It is not surprising that these resemblances should have misled early investigators. For a long period only the external forms of zoophytes were known, and every analogy observed authorized their arrangement with plants.* The discovery of the flowers or seed of corals was yet to be made to prove the identity; and at last, Marsigli, an active explorer of the Mediterranean, came forward with this veritable discovery itself, and published figures of "les fleurs du corail"-the coral blossoms.† Other discoveries followed but it was soon shown, that these flowers, were gifted with the attributes of animal life. This observation is said to have been first made by Ferrante Imperato, a naturalist of Naples, who published his Historia Naturale in 1599. It was however demonstrated independently, as is believed, and more thoroughly, by Peyssonel, who wrote an elaborate memoir on certain species examined by him in the West Indies. But before a transfer of zoophytes from the vegetable to the animal kingdom was generally allowed, the subject was one of warm debate among the philosophers of the day. The animals detected were suspected of being parasites, and pronounced as too inefficient for the production of trees of stone with their spreading branches; while the formation of coral was attributed to a kind of vegetable growth by some, and to mineral aggregation or crystallization by Among the authors who arranged corals with the vegetable kingdom are Dioscorides, Cæsalpin, Bauhin, Ray, Geoffroy, Tournefort, and Marsigli. Marsigli, Physique de la Mer, Amsterdam, 1725. His first observations were made in 1706. See Blainville's Manuel d'Actinologie, p. 14. § Peyssonel's Memoir covers 400 pages of manuscript. It was sent to the Royal Society in 1751, and an abstract of it was read, which appeared in the Transactions for 1753, (vol. x, of the Abridgment.) The Memoir, though for many years supposed to be lost, is still extant in the library of the museum at Paris; and a late notice of it by M. Flourens may be found in the Annales des Sciences Naturelles, 2nd ser., ix, 334, 1838. Dr. J. Parsons made a labored and apparently successful reply to Peyssonel before the Royal Society in 1752, in which he argues ab ignorantia: "It would seem to me much more difficult to conceive that so fine an arrangement of parts, such masses as these bodies consist of, and such regular ramifications in some, and such well contrived organs to serve for vegetation in others, should be the operations of poor, helpless, jelly-like animals, rather than the work of more sure vegetation, which carries on the growth of the tallest and largest trees with the same natural ease and influence as the minutest plant." 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