The Canadian Horticulturist, Volume I / Compendium & Index

by Various

Fig. 15.

We have given this account of the history of this insect, not because it does any injury to our crops of fruit or grain, but because its history illustrates the metamorphoses of many other insects, and any who wish can easily rear it in confinement, and watch the changes it undergoes.

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RISE AND FALL OF SAP.

It is a very commonly received idea that the sap of trees descends in autumn, and when the leaves fall, returns to the roots whence it came in the spring. It does not seem to have once occurred to those who accept this view of the matter, that there would be any difficulty in cramming a quart of water into a pint cup; perhaps, in order to accommodate the sap, the roots are supposed to be as large and capacious as the trunk and head of the tree; in short, that there is as much tree underground as above. Whence the idea sprang into existence that the sap retires to the roots on the approach of winter, it is not now possible to say, but its very general popular acceptation is an evidence of the way in which false views gain currency because some one ventures to make the assertion, and the public mind stops not even to weigh the probabilities, much less to investigate the ground upon which it rests. Surely the time has come when we should cease to accept assertions upon trust, and demand the facts on which they are based. Having these, we can consider the theory, and if it does not suit us we can make one for ourselves.

Now, in this matter of the sap, by which we mean all the fluids which are contained in the interior of a tree, the facts are these: if the trunk be cut in spring, the sap will run out; in summer, autumn, and winter it will not, except under exceptional circumstances. But nevertheless the sap is in motion in the summer and autumn, and winter too; nay, save when extreme cold may for a time interfere with its flow, it is always in motion; and the reason why it runs out of the trunk in the spring is because it is then present in much greater abundance than at any other season of the year. During the summer, when the tree is covered with foliage, the leaves are evaporating large quantities of the fluid parts of the tree into the air, while another portion is being elaborated and converted into the tissues and structure of the tree, producing what we call growth. When the autumn has come, what with the evaporation and solidification that has taken place, the interior of the tree has become comparatively dry, so that the quantity of sap has become so greatly diminished that it no longer exudes when an incision is made. Our readers are, at least many of them, aware that if a branch be cut off from a grape vine in spring when the buds are starting, the sap will run out quite freely, producing what is called bleeding; but if the same branch were allowed to remain until the leaves on the vine have became fully expanded, then if it be cut off no bleeding will take place. The reason is, that the evaporation which is taking place in the leaves has exhausted the supply of sap to such an extent that there is no surplus in the vine to escape in that way.

The leaves being the principal organs of assimilation and perspiration, it follows that when they have fallen off there is no longer much loss of fluid to the tree from these causes. But the power of the roots to absorb moisture from the earth is not diminished by the loss of the leaves; they continue to draw fluid from the earth, and to send it up into the tree. This action continues, except as modified by extreme cold, all winter; the fluids are drawn from the soil by the roots and sent into the tree, and by the time that spring has come the tree is full of fluids and every vessel distended with sap. During the winter we are not able to find sap by cutting the tree, because the process of filling with fluid is gradual.

M. Biot, many years ago, made some very interesting experiments on the flow of sap, and made a contrivance by which the rate of motion could be measured at any season, and showed that there was considerable activity even in winter. He found that the direction in which the sap moved was very considerably affected by frost. When the weather was mild the sap was always ascending; but when it was freezing weather the sap flowed down. This he attributed to the contraction of the sap-vessels by the cold, which forced the sap into the larger vessels which were unaffected by the frost under ground. When, however, the frost was sufficiently severe and continued to reach the roots, then the sap was forced back into the trunk; but when it came on to thaw and the frost left the ground, the sap returned to the roots. Thus we see that, as a rule, the sap is always ascending, and that when it descends it is because it is forced to do so by some temporary cause, and when that cause ceases to act the sap immediately begins to ascend again.

In connection with the supposed ascent of the sap in spring, and growing out of it, is the popular idea that this ascent of the sap is the cause of the expansion of the buds and leaves. It would be nearer the truth to say that the expansion of the buds and leaves was the cause of the motion of the sap. Any of our readers can make the following experiment for themselves, and see the true state of the case. If a tree be cut into or tapped in some of the upright branches near the top very early in the spring, and be again tapped just below the branches on the trunk, and again just above the surface of the ground, it will be found that the sap will flow from the wound that is nearest to the top first, from the one just below the branches next, and last of all from the one near the ground. The reason for this is, that the light and warmth tell first upon the excitable buds at the extremities of the tree, and therefore the sap is set into an accelerated motion that lies nearest to them. The gentleman who first made this discovery came to the conclusion that in the spring the sap of trees descended instead of ascending, but he forgot that the moment the buds begin to expand they draw the sap from the parts nearest to them; this causes the sap just beyond to push upwards to supply the place of that taken up by the buds, and as the buds increase and expand is this absorption increased, and the circle of motion enlarged from the buds downwards.

Were it not true that the sap is constantly ascending, we should lose all our evergreens during the very first winter, for the evaporation that goes on in winter from their leaves would soon season the wood of the tree were it not made good from the roots below; and hence it is that some evergreens are killed by a severe winter while they are small, which would survive without injury had they attained to a greater size; for, being small, the roots have not penetrated to sufficient depth to reach below the frozen ground, and consequently can not draw from the frozen earth in sufficient quantity, nor with sufficient rapidity, to supply the waste by evaporation; whereas when larger the roots will have penetrated quite below the reach of frost, and will be able to draw from the soil sufficient moisture to supply the loss.