specimen of the large shark's tooth, (Carcharias megalodon,) a species occuring also in North Carolina.

One of the most remarkable places near Tampa is a large sulphur spring, eight miles up the Hillsborough river. About one hundred yards up a tributary, the spring boils up and fills a capacious basin. It is white sulphur water coming from the crevices of limestone and from twelve to eighteen feet deep, and so perfectly limpid that the incrustations of sulphur at the bottom are as distinctly visible as if the water were but an inch in depth. Just below the spring great numbers of mullets may be seen scudding in all directions as the boat passes over them, but not a fish of any kind seems to trespass upon the sulphureous basin.

In the fresh water of Manatee river, a large soft-shelled turtle is frequently caught by the settlers who prize it as an article of food. I inquired the number of claws on each foot, and was answered by a man who had recently caught one, that there are five claws to each foot, which verifies the accuracy of Bartram, who describes and figures an unknown species with this character, which must be very distinct from the common soft-shelled tortoise of the south, (Trionyx ferox.)

Since writing the preceding sketch of Tampa bay, I have examined more attentively the casts and shells contained in the limestone. None of these can be identified with any recent shells of Florida or the West Indies. But what has satified me that this rock is geologically older than the Post-pliocene, is the occurrence of an extinct species of land shells, a Bulimus which is silicified and extremely perfect. No similar species of this genus exists in North America. All the other shells in the rock are decidedly of marine origin, and very large masses of a new species of Columnaria abound. It is remarkable that the fossils of this rock have no near resemblance to those either of the Miocene or Eocene, and it will most probably prove to be an upper member of the latter. The local depositions of this formation vary much in their fossil contents. Those of the Santee river in South Carolina, are quite distinct from the group which characterizes the Eocene limestone at St. Stephens, Alabama.

I have obtained casts of a species of Venus; of Nucula, two; Cytherea, one; Natica, one; Bulla, one; Bulimus, one; and two Foraminifera, comprising a Nummulite and a Cristellaria. The latter multilocular shell can be obtained by pounding the rock in

to small fragments, when the shells fall out entire, and some of them can readily be seen without a glass.

The variations in the groups of shells may be in part owing to some difference in geological age, as the Eocene has unquestionably newer and older members, as well as the Pliocene or other formations. But depositions in different depths of water and on different kinds of bottom have also caused local variations in the fossil contents of Eocene rocks. Much light could be thrown upon this subject by a careful investigation of recent species inhabiting different depths of water along the coast. The Miocene, however, could be still better illustrated by this means, because the deep-water species are precisely those which most abound in the Miocene and are most rarely seen upon the beaches. Thus while sailing in the Gulf of Mexico, I watched with interest such specimens from the bottom as the lead brought up, and by this means I obtained a new Corbula, a specimen of Dentalium coarctatum, which I believe to be the first living one found this side of the Atlantic, although it is a common shell in the Virginia Miocene. I obtained also Cytherea elevata and Nucula acuta, which are likewise common in Miocene deposits. All these are deep-water species, living at the depth of eighteen or twenty fathoms, which I never found recent on the beach, and therefore they may fairly be cited as evidence of the fact that such Miocene localities as contain abundance of them, originated in water of similar depth. Other localities of Miocene contain quite different species, and beds of shoal-water origin can often be clearly made out. With regard to the Eocene, no doubt can be entertained that similar causes have produced similar effects. I hope to make out a line of demarcation between the older and newer strata of the Eocene period that may be convenient in the study of this formation. Thus I propose the Nummulite limestone of St. Stephens and Clark county, Alabama, and the fossilliferous bluff at Claiborne, as Lower Eocene, inasmuch as they hold a few species intimately related to, if not identical with, cretaceous forms, and so far as we know, all the species are extinct. The limestone of Savannah river in Georgia, between Savannah and Shell bluff, contains two recent shells, Lithodomus dactylus and Trochus agglutinans, and is evidently a later deposition than the former rocks. I propose to term this, Upper Eocene, and very probably the prevalent limestone of Florida will be included in this division.

This rock extends throughout the peninsula, as far south at least as Tampa bay; and both the east and west shores of this peninsula are covered with a Pleistocene formation of recent species of shells and remains of mammalia. The elevation of East Florida above the sea level is so inconsiderable, that all or nearly all of it must have been submerged at the time the Post-pliocene species were existing, and therefore its elevation was contemporaneous with that of the Keys, which line its eastern, southern, and western coasts.

ART. VI. The Physical Structure of Plants; by D. P. GARDNER, M. D., formerly Professor of Chemistry and Natural Philosophy in Hampden Sidney College, Virginia.

1. THE composition of the earth's atmosphere is the result of great physical laws, which acting in all places produce an uniformity of structure of the highest interest and importance. Whatever cavities or porous systems lie in this medium are subject to its laws, and the same forces which produce the diffusion of gaseous volumes into the air, govern their penetration throughout membranes. The movement into a cavity, and the escape of aëriform matter come to an end after a time, and there remains a similar atmosphere within, and around the exterior. So true is this that whatever gases are confined in porous bodies, pervious tissues, or fissured vessels, will sooner or later acquire the composition of the common atmosphere.

2. Botanists in studying the phenomena of plants, have hitherto been too much attracted by those of the highest development, in which a complex machinery of spiral vessels, laticiferous ducts, and other additional organs exist. The office of these parts will undoubtedly be resolved in time, but unless we first comprehend the general functions, the uncertainty which now exists in vegetable physiology will remain undispelled. The green mildew of damp walls, Chlorococcum vulgare, offers us an instance of an elementary plant, consisting of a single cell, which is nevertheless possessed of all the essential qualities of a vegetable. Within its limited dimensions are stored the forces of growth; the ability to decompose carbonic acid for the production of amylaceous matters-within its one wall the azotized products are formed

and seeds perfected. To enter this organism, gases must penetrate the bounding tissue, and whatever elastic products are formed by chemical decomposition must find exit. But however readily some might admit the porosity of this inferior vegetable, most botanists are disposed to look to a recondite principle, a vitality, for the solution of physiological points. The influence of ordinary penetration in plants has never been proved, although conjectured by Liebig, Dumas, and the more speculative chemists, on the contrary the only direct series of experiments made on the subject by Messrs. Ferrand and Calvert, (Ann. de Chim. &c., August, 1844,) led those gentlemen to the opposite conclusion. They assert, at page 485 of their memoir, that "the bladders of Colutea arborescens are not permeable to the air except in the most limited degree." In the case of Clorococcum the penetration of gases is not only an essential point but nearly the only physical phenomenon, the other processes of growth and decomposition of carbonic acid, &c., being effects of this in a great degree. Now there is little doubt that the organic forces of this plant are those of the whole vegetable kingdom; hence to prove that the structures of other plants is identical with this, is to show that all are reducible in their essential functions to a system, subject to physical penetration, and this is the object before me. I propose to discuss this position under the following heads:1st. The bounding membrane of plants is porous.

2d. The nature of the internal gas of plants.

3d. The action of roots on the gases of the soil fluid. 4th. The absorption of gases by plants is a result of their porosity.

5th. The action of plants upon artificial atmospheres.

I. The bounding Membrane of Plants is porous.

3. The object here is to show that the epidermis is not merely capable of transmitting a gas of particular composition, but that it obeys the theoretical requisitions of a porous membrane. This is not indeed asserted of other plants than those employed, although I believe it to be a general law of the vegetable kingdom, the exceptions being inconsiderable. The bounding membrane of leaves is taken for experiment, because there exists no doubt of the free intercommunication within plants by the cellular spaces and other channels, and because it is obvious that if any SECOND SERIES, Vol. II, No. 4.-July, 1846.

7

vital force or antagonism existed to the operation of the physical laws of penetration it would be situated at the threshold; over the exterior and not in the interior of plants.

4. The experiments were planned with the twofold object of showing that carbonic acid would penetrate into a vessel containing common air, notwithstanding the opposition of a barrier of vegetable epidermis, and secondly that an enclosed atmosphere of theoretical composition would solicit the passage of both carbonic acid and oxygen towards it, and at the same time throw out nitrogen gas. There was some difficulty in obtaining perfect specimens of epidermis from most leaves and the observations were limited to those plants which furnished it readily and were accessible.

5. The experiments.—August 6th, 1845, a tube five inches long and one third of an inch in bore was selected, and one extremity softened and pressed until it presented a thick ring of glass, this was next ground to a plane surface. This tube was depressed in a mercurial vessel to within an inch of the mouth, a portion of the fresh epidermis of the Madeira vine (Bassilla lucida) was then adapted to the ground surface by means of soft wax. The portion of membrane lying over the bore of the tube was circular and nearly a third of an inch in diameter. The tube was now raised up three inches from the trough and suspended by a wire; the membrane sustained the pressure of three inches of mercury without leakage. In about ten minutes sufficient clear limewater was introduced through the column of mercury to reduce its level to that in the vessel, so that the tube contained one inch of atmospheric air at the same pressure as that without, and three inches of limewater, and was closed above by vegetable membrane and below by mercury. Over this arrangement a small bell jar was placed containing atmospheric air with 10 per cent. carbonic acid. The experiment commenced at 1 o'clock P. M., temperature 81° Fah., and at 6 o'clock P. M. the limewater exhibited a distinct pellicle of carbonate of lime.

6. The carbonic acid therefore penetrated the epidermis. This experiment was made with slight modifications with the upper and lower epidermis of the Madeira vine, the epidermis of the cabbage leaf, the Alanthus alata, Chenopodium album, and several species of Sedum which are covered with a tough skin readily removed. Some leaves, as from the Impatiens balsamica,