In vegetables secretion seems to be limited to the preparation of the nutrient fluid by the elimination of certain matters, and, so far as our knowledge extends of the end to be an swered by any act, for the formation of the generative fluids; we do not, in fact, find among vegetables any apparatus set apart for the excretion of matters derived from a change in the constituent particles of the organs once formed. Among animals, again, the apparatus by which this depuration of the system is ac complished is one of the most important of all to the preservation of the individual. Secretion among vegetables is a function much more under the influence of external circumstances than it is among animals ; it is also more sub ject to periodical changes among the former than among the latter, and whilst the function is mostly called into activity by the stimulus of light, heat, &c. in the one, it rather obeys cer tain internal and peculiar stimuli transmitted through the medium of the nervous system in the other.
Like all the other special modes of activity manifested by organized beings, secretion is one of the products of the laws of vitality with the essence of which we are altogether un acquainted.
Besides the secretion of the various gaseous, fluid and solid matters mentioned, vegetables and animals appear in common to possess the power of disengaging certain imponderable light, and electricity.
.Heut.—There has been considerable variety of opinion among physiologists with regard to the extent to which vegetables have the power of maintaining a temperature of their own inde pendently of that of the surrounding media. Nor is this question, in our opinion, yet com pletely set at rest. It is certain that trees in high northern latitudes endure a cold many de grees below zero without injury, whilst in in tertropical countries they are frequently ex posed even in the shade to a heat above that of any animal without perishing ; actual ex periment, indeed, proves that they preserve a temperature intermediate between that of the extreme heat and extreme cold of the diurnal variations of those latitudes in which they are indigenous. This circumstance is explained variously, some attributing it to a vital property in plants to regulate to a certain extent their own temperature, others alleging that it is merely owing to the indifferent conducting qualities of the materials of which vegetables are composed. The thermometer has been seen several degrees below the freezing point of water within the trunks of fir trees, without their vitality being affected ; but it is probable that the constitution of this tribe renders them capable of enduring such a reduction of tem perature with impunity as would prove fatal to other trees with simple watery sap.
On the other hand, it is quite certain that the flowers of many vegetables have the power of disengaging heat, a difference of ten, twenty, and even more than thirty degrees having been observed at sun-rise between the temperature of the atmosphere and that of the flowers of different vegetables in southern latitudes, and the same thing is known to occur, though to a less extent, in northern countries.
It would therefore be unfair, with such facts before us, to deny altogether to vegetables the faculty of disengaging caloric. Arguments, in deed, cl priori, might be adduced to show that they must almost necessarily possess such a property : they are the subjects of incessant change; and one of the most universal of the physical laws involves a change of temperature on any change of constitution.
If the faculty of vegetables generally to secrete or eliminate caloric be doubtful, how ever, it is indisputable that among all animals a little raised above those at the very bottom of the scale, there is an inherent power of gene rating caloric, which in their state of maturity is nearly determinate as regards each particular species. Mammalia and birds have universally the highest temperatures.
' Reptiles or cold-blooded animals, as they are improperly called, have also the power of engendering heat, and of regulating their own temperature : this faculty, however, and the degree of heat they possess at different times, are influenced to a very considerable degree by the heat of the media in which they live. The same statements may be made with regard to fishes. The temperature of these creatures is generally several degrees above that of the water they inhabit; but it also varies with the tem perature of their native element.
Many insects have a very decided power of engendering heat and of regulating their tem perature ; and similar faculties have been de monstrated in the crustacea, the mollusca, and the annelida. These tribes, however, are all very much influenced by the temperature of the media surrounded by which they live.
No great difference is therefore discernible between vegetables and animals in the faculties they possess of engendering caloric and regu lating their own temperature ; the faculty is only much more decided, and possessed to a far greater extent among the more perfect classes of animals generally than among vege tables at large. It may very fairly, in the present state of our knowledge, be ascribed as a common property.