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ARTIFICIAL OBSTACLES TO SAP.

Some curious experiments upon this subject were contrived by Mr. N. Niven (Gardeners' Magazine, vol. xiv.). In one case, he divested the stem of a tree of a deep ring of bark, and of the first twelve layers of wood below it (Fig. XII.); nevertheless the

Fig. XII.-Ringed tree, having wood removed as well as bark.

From the exposed

tree continued to live and be healthy. surface of the wood no sap made its appearance, except from a cut which had been inadvertently made with the saw on one side, to the depth of, perhaps, five or six layers of wood beyond the twelve actually removed. From that cut a flow of sap took place, and continued to run during the whole of the season in which the operation was performed. In this case, the sap cannot have ascended exclusively by the alburnum, but must have chiefly passed through the central wood.

In another case, by making four deep and wide incisions into the trunk of a tree (Fig. XIII.), and removing the centre, the upper part of the trunk was placed upon four separate pillars of bark

ARTIFICIAL OBSTACLES TO SAP.

49

and alburnum; and the tree upon which the operation was performed continued to live for two years, after which it was not observed. In this instance, no doubt can be entertained

that the whole of the sap was directed into the four pillars, after passing through which it was conveyed laterally both in ascent and descent until the whole system was again filled.

The cause of the flow of the sap appears to be the attraction of it by the leaves, which continually diminish its quantity; and the necessity that the sap abstracted should be replaced by a further supply sent upwards from the roots. The consequence of this is, that sap always begins to flow at the ends of branches, a circumstance which has led to the erroneous idea that it proceeds from above downwards through the alburnum. The flow of the sap must not, however, be confounded with the motion of the sap, which takes place in the winter as well as

E

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30

BLEEDING FROM ROOT AND STEM.

in the summer, and is a mere impletion of the system, caused by the absorbing force of the roots, unaffected by the exhalation of the leaves.

Occasionally they discharge sap in such abundance from the wound that they are said to BLEED; and when the sap is red, as in Pergularia sanguinolenta, the discharge has the appearance of being identical with the bleeding of animals. When trees begin to grow in the spring, sap flows abundantly from their wounds; it may even be collected for fermentation, as occurs to the Birch-tree, whose juice, obtained by tapping, in the early spring, becomes converted into a sparkling wine; or, by boiling, the saccharine matter dissolved in sap may be collected in abundance, as is the case in North America with the Sugar Maple. If allowed to continue for too long a time such bleeding causes incurable debility or death. The roots of a tree will bleed as much as the stem; and with the same consequences. A case is mentioned by the Hon. Jas. Stuart Wortley of a very fine. Birch-tree, whose roots were cut through in making a new walk near it. They were about five in number, and averaged about an inch and a half in diameter, and continued bleeding so incessantly for a fortnight, that the walk at the end of that time stood in puddles, and the sap still bubbled up through the gravel. The same circumstance was observed in lowering the ground near a large Walnut-tree, when some great roots having been cut through, so much bleeding took place in consequence that the tree died.

A third occurrence of the same nature has been recorded by Mr. Spencer, gardener to the Marquess of Lansdowne, at Bowood. In forming a new walk, he had occasion to cut through three large roots belonging to an adjoining Beech which remained exposed. Some time about the middle of March he observed the roots were bleeding considerably, and they continued to do so till the end of April, the flow being materially influenced by the state of the weather. By the beginning of April the bleeding was sufficient to saturate the walks. On examining the roots with an ordinary microscope, he observed the discharge proceeded from the whole of the exposed cells through the section; but, from the larger diameter of the vessels towards the exterior

CAUSE OF BLEEDING.

51

of the root, the bleeding, as a natural consequence, was greatest at that part. He also remarked that bubbles of air were frequently formed on the cut surface, evidently showing that some kind of gas was present, either in the sap or in the cells. The discharge was perfectly visible to the naked eye, and in bright weather the microscope enabled him to see distinctly the downward passage of the sap, through all the root cells.

Such cases are doubtless much more common than is supposed. The cause of the phenomena was in part demonstrated more than a century since by Hales. In discussing the question of the circulation or non-circulation of the sap, this great experimentalist uses the following words;— "We see in many of the foregoing experiments, what quantities of moisture trees do daily imbibe and perspire. Now the celerity of the sap must be very great, if that quantity of moisture must, most of it, ascend to the top of the tree, then descend, and again ascend, before it is carried off by perspiration. The defect of a circulation in vegetables seems in some measure to be supplied by the much greater quantity of liquor which the vegetable takes in, than the animal, whereby its motion is accelerated; for by Experiment 1st, we find the Sunflower, bulk for bulk, imbibes and perspires seventeen times more fresh liquor than a man every twenty-four hours. Besides, Nature's great aim in vegetables being only that the vegetable life be carried on and maintained, there was no occasion to give its sap the rapid motion which was necessary for the blood of animals. In animals, it is the heart which sets the blood in motion, and makes it continually circulate; but in vegetables, we can discover no other cause of the sap's motion but the strong attraction of the capillary sap vessels, assisted by the brisk undulations and vibrations caused by the sun's warmth, whereby the sap is carried up to the top of the tallest trees, and is there perspired off through the leaves: but when the surface of the tree is greatly diminished by the loss of its leaves, then also the perspiration and motion of the sap is proportionably diminished, as is plain from many of the foregoing experiments; so that the ascending velocity of the

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CAUSE OF ASCENT OF SAP.

sap is principally accelerated by the plentiful perspiration of the leaves."

The sap then ascends in consequence of an attracting force exercised from above downwards by the foliage of plants. But it is evident that this is only a partial explanation of the phenomenon; for it does not account for the ascent of sap in winter when leaves are absent. In order to explain that fact we must have recourse to the action of endosmose, a force the effect of which is to produce propulsion. A tree may be assumed to be a combination of hollow tubes freely communicating with each other, and enclosed in a skin through which fluids are capable of being absorbed on the one hand and expelled on the other. If we conceive a body of this kind, in which the tubes are nearly empty, to have its lower extremity plunged in water, the absorbing power of the skin at that part will begin to introduce the water into the interior, and this continuing to go on for a sufficient time, the tubes must necessarily become at last filled with water rising upwards from below. To effect this, no attracting force at the upper end of the cylinder was necessary; every particle of water which was absorbed by the lower end, having driven before it a corresponding volume of the water previously existing in the apparatus. Under the influence of this operation the tubes would in time become full, and if unelastic the introduction of more water would be impossible. But if such tubes and the skin that encloses them were elastic and extensible, then any such further quantity of water might be introduced as the apparatus could receive without bursting. If we then suppose that the one end of the apparatus were cut open the sides of the tubes would collapse, and the water would be forced out till there was no more left than the tubes held in their original unstretched condition. A tree is just such an apparatus. Its tubes are nearly empty at the fall of the leaf. During winter the roots absorb water from the soil and fill the tubes again. By the arrival of spring they are filled almost to bursting, and then if the stem is cut it bleeds; or if the roots are cut they bleed. Bleeding ceases as the leaves unfold. The Vine, the Walnut, the Birch, are all as incapable of bleeding as other trees when