Experiments in Connection with the Circulation of Sap in Trees.— Variety of Sap-exuding Trees.—Non Sap-yielding Species.—The Influence of Climate on Flow of Sap.—Composition of Sap, to What Due. — Distinctive Characteristics of Sap-yielding Trees Demonstrated.—Effect of the Temperature of Soil and Atmosphere on Sap-flow.—Principal Ingredients of Sap.—Daily Meteorological Observations and What they Prove.—Explanations on the Alternations of Sap-flow.—The Observations of Biot and Nevins, and What they Determine.—The Opinion of Mr. Hubbard Confirmed by Experiments.—The Absorbent Power of Roots.—Development of Leaf and Flower, How Influenced, and Origin of their Vitality.
A seeies of experiments made by Professor "W. S. Clarke, President of the Massachusetts Agricultural College, throws much light on a subject which has hitherto remained in great obscurity—the circulation of sap in trees—and promises an understanding of many things connected with pruning and transplanting which have hitherto been veiled in obscurity. Unable, from want of space, to present our readers with the full report, we endeavor to condense the material portions into a brief space. The familiar facts—that sap flows from wounds in certain trees in the spring, that from the sap of the maple sugar is obtained, and that the peculiarities of the season affect the quality and quantity of the flow, suggested these experiments, whose object was to determine the amount, pressure, and composition of sap which might be obtained from different species of woody oxogens. The great majority of trees and shrubs, it was found, do not at any season of the year bleed from wounds in the wood, and but few of the species which, in our northern latitilde, exhibit this phenomenon at all do so when clothed in foliage. The striking and extraordinary differences thus evidenced are not accounted for by any peculiarity of structure or habitat. The soft and spongy wood of the willow and elm, growing in moist ground, seem specially suited to absorb and pour forth water before the expansion of their leaves or flowers in the spring; but examination shows that they contain no unusual amount of sap at that time. Of more than sixty species of trees and shrubs tested by Professor Clarke, only six—Betula, which includes the birch; Acer, the maples; Vitis, the vines; Ostrea, the hornbeam; Juglans, walnuts; and Ca-rya, the hickories—showed any tendency to bleed. The genus Garya exudes but very little, and possibly Fagus, the beech; and Carpinus, the hop hornbeam, may do the same, though no satisfactory test was applied.
It was found that each species had its own time of beginning the flow of sap; that the flow then steadily increased in quantity and force until the maximum was reached, when it gradually declined; and that the composition of the sap of the several species differed remarkably, both according to the date of the flow and the time of its beginning.
This singular periodicity demonstrates that the absorption of water by the rootlets is not caused by osmose or any other merely physical force, but is the result of that specific life which imparts to every plant its distinctive characteristics.
The sugar maple, which begins its flow in October, reaches its maximum about the first of April, and ceases about the first of May. The black birch begins the last of March, reaches its maximum in a single month, and stops entirely about the middle of May. The wild summer grape-vine commences the first of May, arrives at its maximum by the twenty-fifth of the same month, and ceases early in June. Differences in the season of flowing are of course accompanied by corresponding differences in the temperature of the soil and atmosphere, as also in the chemical condition of the sap. The principal ingredient of maple sap is cane sugar; of birch sap, grape sugar; and of Tine sap, mucilage or gum.
But why do we find cane sugar in the maple, and not in the birch ? and why only gum in the vine 1 Possibly because these several transformations of the starch (which descended to the root of the plant and was deposited in its cells, or in those of the stem, as the result of the previous season's growth) require different periods of time. The maple is the only one gorged with sap during the six months which intervene between the fall of the leaf and the beginning of spring growth. This affords ample time for the necessary chemical changes, and may account for the fact that the maple is the only indigenous tree from which crystallizable cane sugar can be profitably extracted. Birches are next in order. Being filled with sap for several weeks before a bud begins to expand, we may reasonably expect to find in them the formation at least of grape sugar; and in the north of Europe a sweet syrup is obtained from their sap by evaporation. At last the vine. The beginning of the motion of its sap is deferred until about the first of May, at which time it seems to contain no sugar of any kind. Three weeks later it acquires a sweetish taste, and we may then find a trace of grape sugar. At this period the beginning of vegetable growth is attended by the rapid exhalation of the water of the crude sap and the assimilation of its gum in the formation of cellulose, and this is precisely the transformation which ordinarily occurs in plants at the beginning of the vegetating season. A careful comparison of the daily weight of the sap from several sugar-maple-trees with the meteorological observations of the same period, conclusively proves that while the general flow corresponds with the season—rising to a maximum and declining—the daily and hourly flow varies with the weather. Steadily and severely cold and uniformly warm and foggy weather are the most unfavorable, while the best sap days are bright and warm, preceded by freezing nights.
The variations of temperature which affect the flow of maple sap are most likely to occur when the ground is covered with snow, because the heat of the sun during the day cannot then overcome the cooling influence of night. The most probable explanation of the effect of these alternations appears to be that the contracting influence of the cold drives sap from the outer tissue of the tree into the heart-wood of the higher parts of the trunk. Meanwhile absorption goes on as usual underground, and thus, when relief is afforded by the expansive influence of the sun, the sap rushes again to the surface and flows abundantly. This explanation is confirmed by the observations of Biot, in France, as to the poplar; and by Nevins, in Ireland, as to the elm. To determine whether sap would flow from the heart-wood of a sugar-tree a piece of gas-pipe was driven to a depth of six inches. The flow was regular and long-continued, but not abundant. From another tree a piece of bark five inches wide and three inches high was removed, and a piece of sheet-iron driven into the bark below to catch the sap which flowed very profusely but stopped very early. From the first tree the sap flowed eleven days longer than from the last, but the latter yielded twelve pounds more of the fluid.