CIRCULA.TION OF THE SAP. The circula tion of the sap in plants was, until within a few years past, a source of wonderment, and various theories, some of them ridiculous enough, were advanced by learned (?) rnen of the scholastic class. Even now, comparatively little is posi tively known as to some of the processes by and through which the plant lives and grows. It is very generally supposed that there is an upward and downward flow of sap in trees dependent upon the season. It is even now taught that the crude sap is taken in by the roots, ascends through the wood to the leaves, is then evapor ated of its water to a certain degree, and elabor ated by functional offices of the foliage, and thence descends through the inncr bark, to nourish these tissues along its course, and form new growth. Many intelligent persons believe that the flow of sap is, like the current in a river, continuous. This latter proposition is so far true that when the leaves are transferring moisture steadily, and the supply of moisture at the root is sufficient, the flow of sap is steady. The flow of sap in thc maple has often been quoted to prove this, yet it certainly did bother the savants, as well as the simple, to know how the sap got up through frozen roots in frozen earth. The facts in relation to the exudation of sap in the maple, and some other trees, in the spring, when wounded, is to be accounted for in the most mple manner. It is nonsense to suppose there Is any regular flow of sap, in a tree, leafless, its buds entirely dormant, its hark impervious to moisture and its roots wrapped in frost. A maple, for instance, will not bleed, if wounded, when the temperature has fallen below a certain point. Thearand, in Saxony, found, in direct antagonism to the popular belief, that the pro portion or water, both in the live hark and the wood of trees, varies considerably, at different seasons of the year, the range in beeches felled, being from thirty-five to forty-nine per cent; the greatSr proportion of water in the wood of fresh felled trees being in the months of December and January, and the least quantity in May, June and July. In the bark the greatest quantity was found from March to May; but, in the bark, great irregularity occurred. The stem of the maple is therefore fully charged with water at the time of sugar-making. Whenever the sun's rays are of sufficient strength to create heat sufficient to expand the moisture and air of the tree, sap will flow. When the heat declines, the sap ceases to flow. That water is produced by chemical action in the trunk is by no means improbable. That the supply originally ca.me from the roots is certain; and that a continual supply is passed up from the roots during the sugar-making season is as certain and proved from the fact that, although the ground be frozen, the roots will not be, neither will the earth next them be frozen; and thus they furnish a passage for the flow of sap. The absorbing power of these deep roots remaining constant from day to day, in con sequence of the uniform temperature, they fur nish an equable quantity to supply the exhaustion -consequent upon an increase of temperature in the trunk from the sun's power. Theis, if this heat were constant the flow of sap would be equable, only limited by the supply from the roots. When the foliage expands, the flow from incisions .of course will cease, since there is then an outlet for the escape of vapor through the pores of the leaves. When this takes place we have a differ ent and normal flow of sap. Hence, we easily see why the vine bleeds profusely when cut before the expansion of its leaves and immedi .ately ceases when the buds expand and the leaves perform their normal functions. In the exuda tion of sap through the effect of heat on the air, and on the moisture, contained in the cells of the wood, the temperature of the atmosphere is not always a sure indication of that of solid bodies. Since these take in, retain and increase in tempera ture, many degrees above that of the air. This Is well known in the great heat of the soil, of iron, etc., when long exposed to solar influence in sum mer. Thus in trees, having a sunny exposure, the sap is earlier and crops are faster advanced in warm (sandy) soils, if moisture enough be pres ent. Hartig has shown one mode by which
changes of temperature in the trunks of trees influ ence the flow of sap. The wood-cells contain. not only water but air. Both are expanded by heat and both contract by cold. Air, especially, under goes a decided change of bulk in this way. Water expands nearly one-twentieth in being warmed from 32° to 212°, and air increases in volume more than one-third by the same change of temperature. When, therefore, the trunk of a tree is warmed by the sun's heat the air is expanded, exerts a pressure ou the sap, and forces it out of any wound made through the bark and wood-cells. It only requires a rise of temperature to the extent of a few degrees to occasion from this cause alone a considerable flow of sap from a large tree. A very conclusive experiment was conducted by Duchartre. He passed the stem of a vine, situ ated in a grapery, outside the house and back again, so that a middle portion of the stem was exposed to a winter temperature ranging from 18° to 10° Fahrenheit, while the remainder of the vine, in the house, was surrounded by an atmosphere of 70° Fahrenheit. The buds within developed vigorous ly, but those outside remained dormant and did not open sooner than buds upon another vine whose stem was all out of doors. That sap passed through the cold part of the stem was shown by the fact that the interior shoots sometimes wilted but again recovered their plumpness which could only happen from the partial suppression and renewal of a supply of water through the stem. At the conclusion of the experiment Payen examined the wood of the vine and found the starch, which it originally contained, to have been equally removed from the warm and the exposed parts. In this experiment, curiously enough, whenN the leaves of the vine wilted, it was at night and in the early morning, but recovered during the heat of the day, even when the temperature of the air outside was freezing, Thus we see that the diminished power of the stem to trans mit moisture, was in consequence of cooling at night, but during the day it absorbed and stored heat enough to cause the leaves inside the con servatory to regain their turgid or succulent con dition. Prof. S A. Johnson, in How Crops Grow, says the conditions under which sap flows are various, according to the character of the plant. Our perennial trees have their aninaal period of active growth in the warm season, and their vegetative functions are nearly suppressed during cold weather. As spring approaches the tree renews its growth, and the first evidence of change within is furnished by its bleeding when an opening is made through the bark into the young wood. A maple, tapped for making sugar, loses nothing until the spring warmth attains a certain intensity, and then sap begins to flow from the wounds in its trunk. 'The flow is not constant, but fluctuates with the thermometer, being more copious when the weather is warm, and falling off or suf fering check altogether as it is colder. In rela tion to the different kinds of sap, Prof. Johnson says: It is necessary to give prominence to the fact that there are different kinds of sap in the plant . The cross section of the plant pre sents two kinds of tissue, the cellular and vascu lar. These carry different juices, as is shown by their chemical reactions. In the cell-tissues exist chiefly the non-nitrogenous principles, sugar, starch, oil, etc. The liquid in these cells, as Sachs has shown, commonly contains also organic acids and acid-salts, and hence gives a blue color to red litmus. In the vascular tissue alburninoids preponderate, and the sap of the ducts commonly has an alkaline reaction towards test papers. These different kinds of sap are not, however, always strictly confined to either tissue. In the root-tips and buds of many plants (maize, squash, onion) the young (new-formed) cell-tissue is alkaline from the preponderance of alburninoids, while the spring sap flowing from the ducts and wood of the maple is faintly acid.