An introduction to physiological and systematical botany/Chapter 9
OF THE SAP, AND INSENSIBLE PERSPIRATION.
The sap of trees, as has been mentioned in the last chapter, may be obtained by wounding a stem or branch in spring, just before the buds open, or in the end of autumn, though less copiously, after a slight frost; yet not during the frost. In the Palm-trees of hot countries, it is said to flow from a wound at any time of the year. It has always been observed to flow from the young wood or alburnum of our trees, not from the bark; which agrees with Mr. Knight's theory.
A common branch of the Vine cut through will yield about a pint of this fluid in the course of twenty-four hours. The Birch, Betula alba, affords plenty of sap; some other trees yield but a small quantity. It flows equally upward and downward from a wound, at least proportionably to the quantity of stem or branch in either direction to supply it. Some authors have asserted that in the heat of the day it flows most from the lower part of a wound, and in the cool of the evening from the upper: hence they concluded it was ascending during the first period, and descending in the latter. If the fact be true, some other solution must be sought; nor would it be difficult to invent a theory upon this subject: but we rather prefer the investigation of truth on more solid foundations.
This great motion, called the flowing, of the sap, which is to be detected principally in the spring, and slightly in the autumn, is therefore totally distinct from that constant propulsion of it going on in every growing plant, about which so much has been said in the preceding chapter, and which is proved by taking an entire herb of any kind that has been gathered and suffered to begin to fade, and immersing its root in water. By absorption through the sap-vessels it presently revives, for those vessels require a constant supply from the root.
This flowing of the sap has been thought to demonstrate a circulation, because, there being no leaves to carry it off by perspiration, it is evident that, if it were at these periods running up the sap-vessels with such velocity, it must run down again by other channels. As soon as the leaves expand, its motion is no longer to be detected. The effusion of sap from plants, when cut or wounded, is, during the greater part of the year, comparatively very small. Their secreted fluids run much more abundantly.
I conceive therefore that this flowing is nothing more than a facility in the sap to run, owing to the peculiar irritability of the vegetable body at the times above mentioned; and that it runs only when a wound is made, being naturally at rest till the leaves open, and admit of its proper and regular conveyance. Accordingly, ligatures made at this period, which show so plainly the course of the blood in an animal body, have never been found to throw any light upon the vegetable circulation. This great facility in the sap to run is the first step towards the revival of vegetation from the torpor of winter; and its exciting cause is heat, most unquestionably by the action of the latter on the vital principle, and scarcely by any mechanical operation, or expansive power upon the fluids. The effect of heat is in proportion to the degree of cold to which the plant has been accustomed. In forced plants the irritability, or, to use the words of a late ingenious author, who has applied this principle very happily to the elucidation of the animal economy, excitability, is exhausted, as Mr. Knight well remarks, and they require a stronger stimulus to grow with vigour. See. p. 91. Hence vegetation goes on better in the increasing heat of spring than in the decreasing heat of autumn. And here I cannot but offer, by way of illustration, a remark on the theory advanced by La Cepede, the able continuator of Buffon, relative to serpents. That ingenious writer mentions, very truly, that these reptiles awake from their torpid state in the spring, while a much less degree of heat exists in the atmosphere than is perceptible in the autumn, when, seemingly from the increasing cold, they become benumbed; and he explains it by supposing a greater degree of electricity in the air at the former season. Dr. Brown's hypothesis, of their irritability being as it were accumulated during winter, offers a much better solution, either with respect to the animal or vegetable constitution. For the same reason, it is necessary to apply warmth very slowly and carefully to persons frozen, or even chilled only, by a more than usual degree of cold, which renders them more susceptible of heat, and a temperate diet and very moderate stimulants are most safe and useful to the unexhausted constitutions of children. The same principle accounts for the occasional flowing of the sap in autumn after a slight frost. Such a premature cold increases the sensibility of the plant to any warmth that may follow, and produces, in a degree, the same state of its constitution as exists after the longer and severer cold of winter. Let me be allowed a further illustration from the animal kingdom. Every body conversant with labouring cattle must have observed how much sooner they are exhausted by the warm days of autumn, when the nights are cold, than in much hotter weather in summer, and this is surely from the same cause as the autumnal flowing of the vegetable sap.
The sap, or lymph, of most plants when collected in the spring as abovementioned, appears to the sight and taste little else than water, but it soon undergoes fermentation and putrefaction. Even that of the vine is scarcely acid, though it can hardly be obtained without some of the secreted juices, which in that plant are extremely acid and astringent. The sap of the sugar maple, Acer saccharinum, has no taste, though according to Du Hamel every 200lb. of it will afford 10lb. of sugar. Probably, as he remarks, it is not collected without an admixture of secreted fluids.
As soon as the leaves expand, insensible perspiration takes place very copiously, chiefly from those organs, but also in some degree from the bark of the young stem or branches. The liquor perspired becomes sensible to us by being collected from a branch introduced into any sufficiently capacious glass vessel, and proves, for the most part, a clear watery liquor like the sap, and subject to similar chemical changes. It is observed to be uniform in all plants, or nearly so, as well as the sap, except where odorous secretions transude along with it. Still there must be a very essential difference between the original sap of any plant and its perspiration, the latter no longer retaining the rudiments of those fine secretions which are elaborated from the former; but that difference eludes our senses as well as our chemistry. The perspiration of some plants is prodigiously great. The large Annual Sunflower, Helianthus annuus, Gerarde Emac. 751. f. 1, according to Dr. Hales, perspires about 17 times as fast as the ordinary insensible perspiration of the human skin. But of all plants upon record I think the Cornelian Cherry, Cornus mascula, Fl. Græc. t. 151, is most excessive in this respect. The quantity of fluid which evaporates from its leaves in the course of 24 hours, is said to be nearly equal to twice the weight of the whole shrub. Du Hamel Phys. des Arbres, v. 1. 145.