by the inferior wall of the "dorsal tube," v in the cut.

The extremital portions of the axes in both, manifest more or less distinctly the action of flexion in preserving the axial elements separate, since, in the necks and tails of both, there still remains more or less of mobility. In

A, transverse section through the "dorsal tube" of Panocthus tuberculatus, reduced, after both, the union of the verBurmeister. B, the same through the dorsal tebræ has resulted in a simregion of a salt-water terrapin (Malacoclemmys),

original. C, side view of dorsal vertebra of ilar disposition of the latEmys europæa, after Bojanus. eral foramina for the exit of the spinal nerves, that is, these open midway of the length of the vertebræ, piercing the lateral wall of the dorsal tube, and not passing out laterally between the bony arches of the neural canal, as happens in other vertebrates (see A, a, B, a and C, a, of cut). The reduction of the centra in the higher tortoises is, as should be expected, much more manifest than in the lower forms, and the union with the carapace, though not extensive, is manifested in the armadilloes by sutural union in the lumbar region. Similar structural alterations, which are believed to be similarly due to alterations in the mechanical relationship of the skeletal elements, are to be observed in the sacra of birds and mammals where the ilia have been greatly elongated so as to prevent lateral flexures of this portion of the column. The ribs in turtles have been involved in the dermal ossifications, and are therefore, as should be expected (B, b and C, b), united to the vertebral axis by suture; this is not the case in the Hoplophorida, where, owing to the preservation of the more highly specialized mammalian respiratory apparatus, the vertebro-costal articulations are still preserved, with loss in large measure of movement in an outward direction of their sternal ends. The costal movement was probably from behind forwards, with return, since the only articulation of the thoracic axial skeleton which is preserved, is that between the twelfth dorsal vertebra and first lumbar. A priori we should expect the phylogenetic history of the vertebral axes of the order

1 A similar degeneration and anchylosis of the centra is observed in the cervical vertebra of Dipodomys, as I have recently ascertained from a specimen sent me by Dr. Coues.

of Chelonians to agree with ontogenetic history or embryology of the individuals of the highest family. This, so far as I have been able to make investigations, proves, in a measure, to be the case, for it is observed that that portion of the vertebral axis included within the carapace in a young Cistudo has the vertebral centra more nearly of the character of the same parts of the lowest turtles, which approximate in the development of their centra to the normal or usual types of vertebrates with segmented axes. The degeneration of the vertebral axis in Cistudo into a mere tube, with exceedingly thin walls for the lodgment of the spinal cord, may probably be regarded as an instance of Cænogenesis.

These observed coincidences, it is believed, are neither accidental nor designed by an active cause external to these organisms or their cosmic environment. I would rather believe that the structures, so far as they have been evolved in parallel or similar ways, are the results of like forces conditioning growth and nutrition in definite modes and determinate directions. The manner of incidence of the modifying forces being in all cases determined by the voluntary actions of the organisms; the actions in turn are determined by the degree of intelligence of the animal manifesting them.

The origin of dermal ossifications is to my mind rationally explained by supposing the bioplasm of each dermal cell as sensitive and irritable to rude or violent external impacts, which, oft repeated, act as stimuli of growth force, determining certain tracts of these cells as the nidus within which osseous' particles eventually appear as nuclei of the future defensive dermal bony system. This happens in the true skin and not in the corneous epiderm, which is still retained in more or less rudimentary condition in both Chelonians and Armadilloes, though it is not to be forgotten that in the toothless old-world Edentata it is the epiderm which becomes the defensive covering. This thickening, though not depending upon peculiar movements of portions of the body, as in the origin of hoofs, horns, etc., of other forms, depends upon the motion of the whole body mass, during which the hurtful stimulating impacts with the environment take place; so that the rationale of the origin of dermal ossifications is finally resolved into terms of osteoblasts and animal motion. The likeness of the. process of the evolution of a defensive osseous or corneous derm, as sketched above, to the process of reparation in wounds is very

great, indeed, in no essential are they different, except that the former usually goes forward in a bilaterally symmetrical way, while in the latter it most frequently does not.

The preceding facts and considerations embrace what may be regarded as the complementary principle demonstrating the mechanical theory of axial segmentation or origin of vertebræ, as proposed by Spencer, since it must be allowed, that if segmentation is due to flexures of the vertebral axis, conversely, union, coössification of segments, is due to their absence, because opposite conditioning causes must produce opposite effects in two things respectively so conditioned.

IN

:0:

ON THE TRANSPIRATION OF PLANTS.1

BY J. M. ANDERS, M.D., PH.D.

N looking over the literature of the subject, one is surprised to find how little definite knowledge we possess in regard to the process of plant transpiration. When the importance of the subject is considered, there would seem to be no explanation for this apparent omission of research.

It has been pretty well established that transpiration is produced and modified by influences acting from without, and by the structural peculiarities of the plant. Most important among the former modifying agencies are sunlight, wind, dew point and temperature; and among the latter circumstances is to be mentioned, more particularly, the nature of the epidermal tissue. The precise connection between these various conditions and the amount of water evaporated has not been investigated to any considerable extent; and the most important question, viz.: the amount of liquid ordinarily transpired by different plants has, also, hitherto been quite as sadly neglected.

A few bare statements are made in relation to the quantity of transpiration (Gray's Structural and Systematic Botany). A sunflower 31⁄2 feet high, with a leaf surface of 5616 square inches, when exposed to the air, evaporated from 20 to 30 ounces in twelve. hours, being seventeen times as fast as man exhales. A seedling apple tree, with leaf surface amounting to 112 square feet, evapo1 The Geo. B. Wood prize essay, 1877, read before the Society of the Alumni of the Auxiliary Dept of Medicine, University of Pennsylvania.

rated at the rate of 9 ounces per diem, and a vine with twelve square feet of leaf surface, transpired at the rate of 5 to 6 ounces per day. The sunflower during a dry night lost 3 ounces, but nothing on a dewy night. The method adopted in these experiments is not referred to by him.

Balfour, in his work on botany (page 457), refers to the investigations of Woodward, giving some of the results of this observer. Woodward took plants, and, having immersed their roots in water, placed them in the light for more than a month. He noticed the quantity of water absorbed and the amount transpired (making allowance for extraneous evaporation), and showed that the greater quantity of the water absorbed was again given off by the leaves.

It is questionable whether results thus obtained are to be relied upon, inasmuch as these plants must have been placed under unnatural conditions and influences, by allowing their roots to rest in pure water; for it is a known fact that certain plants (Calla Æthiopica, for instance) can be made to distill the water in drops from their leaves, if too abundantly supplied to their roots.

Curiously enough, in every instance in which the methods adopted have been detailed, the objectionable circumstance of placing the plants in a very unnatural state while experimenting upon them has obtained. Reference is here made only to experiments on entire plants. The results of the observations of Garreau (Annales des Sciences Nat. 3d Ser. Bot. xiii. 355) on the transpiration from leaves should, doubtless, be accepted as reliable, if we consider the means employed. This observer estimated the amount of exhalation by collecting it by means of chloride of calcium, placing the leaf between two bell-jars, one applied to its upper and the other to its under surface. His conclusions were:

"1. The quantity of water exhaled by the upper and under surfaces of the leaves is usually as I to 2, I to 3, or even I to 5 or more. The quantity has no relation to the position of the surfaces, for the leaves when reversed gave the same results as when in their natural position. 2. There is a correspondence between the quantity of water exhaled and the number of stomata. 3. The transpiration of fluid takes place in greater quantity on the parts of the epidermis where there is least waxy or fatty matter as along the line of the ribs."

Among the reported results that my eye has been able to reach, the foregoing only are considered worthy of special notice. sibly some have escaped notice.

The present experiments have been performed more especially with the view of ascertaining, as nearly as possible, the amount of water evaporated by plants in a healthy, natural state, and, also, to determine the connection between the meteorological conditions and variations and the nature of the cortical tissue, and transpiration. The importance of keeping the plants in a perfectly normal state while being experimented upon was called attention to by Prof. Rothrock when lecturing on the subject of evaporation from plants. To accomplish this it was suggested by him at the time that something impervious to moisture be adjusted to the receptacle in which the plant had previously been growing, fitting the same accurately to the base of the stem, the object of it all being to prevent any evaporation from the vessel or earth in which the plant was situated, so that all evaporation would be from the plant itself above the ground. The plant was now to be weighed at stated intervals and the loss of weight in any given time would represent the weight of the liquid transpired. It is evident that this, with properly balanced scales, would show exactly the quantity evaporated, save the slight increase in weight of the plant by the gases derived from the air which it fixes in the time of one experiment. This certainly must be regarded as extremely small when we reflect that plants return to the atmosphere the greater portion by volume of the gases absorbed by them. The circumstance of plants gaining slightly in the course of a day by the gases they fix from the air, it will be observed, is not calculated to favor an over-estimate of the quantity transpired.

The means employed to accomplish these ends were as follows:

A piece of good rubber cloth of sufficient size was taken and its narrower border tucked up neatly around the base of the stem of the plant and secured by means of an elastic cord. The rubber cloth was then allowed to drop down over the vessel in which the plant was situated, the portion of the cloth underneath the pot gathered up and brought to one side of its base, and after giving it a few twists in one direction so as to insure its close application to all parts of the pot, the twisted portion was well wrapped and tied off by means of a cord so as to keep it in this condition. This