This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
The last number of the Gardener's Monthly contains a very interesting article from Yardley Taylor on the "Circulation of the Sap," from which we make the following extract:
"The theory of a downward flow of sap, as supposed by the first writers on vegetable physiology, is giving way, and the writers of the present day assert that there is no such flow. (See article 'Botany' in the 'New American Encyclopedia,') Theoretical writers, or those who copy from former ones, often advance the same theory; while practical men, who base their theories upon their observations of nature, reject it Nurserymen who practice grafting, know that success can be had by inserting the graft so that the inner bark of the graft of the stock shall be in contact, so as to admit of the sap from between the bark and the wood passing upward into the graft between its bark and wood. There can be no other way for growth, as the granules of new wood only form between the bark and sap-wood, no where else, and this material for growth must come from below, for there are no leaves above to prepare the matter for assimilation, as the downward theory supposes. Budding, as practiced by nurserymen, is equally conclusive; there the bud only rests upon the sap-wood beneath the bark, and in no contact with the pores of the wood at all, only with the pores between the bark and wood; and if the top of the stalk is cut off, as is usual, it grows at once, though there is no part above from which a downward flow can come.
Many more facts might be mentioned, all leading to the same conclusion, but these, so well known to all who are acquainted with grafting or budding, may suffice; indeed, the evidence appears to me to be so conclusive, that I can see no reason whatever for a downward flow. It appears to me to be too much of a roundabout way to attain an object, to be consistent with what we know of the simplicity of nature's laws; they are always direct, and to be admired for their simplicity.
"If, then, there is no downward circulation, as I confidently believe there is not, the question recurs, 'what is the true theory of the circulation of the sap?' This, to my mind, is very simple and plain; whether I can make it appear plain to others, remains to be seen. Let us recur to facte generally admitted, and base our theory there. All writers on vegetable physiology, I believe, admit that water from rain and snow containing matters for growth is imbibed by the roots, principally by the spongioles or small rootlets, though I have little doubt that even the large roots imbibe some, as their bark is spongy. Among these matters, carbonic acid gas is prominent, and it is generally believed that a portion of this gas is imbibed by the leaves. This gas is considered the only source from which the carbon of the plant can be derived. Carbon, we know, is deposited in growth, and chemists tell us that oxygen is given off in the daytime. Other matters, as potash, etc., are believed to be carried into the plant in the same way; of these facts there is little dispute.
"Carbonic acid gas, however, must be decomposed, and we know that it is, but by what means, there is difference of opinion. Sunlight has been considered the agent of this decomposition, but of this there may be doubts. If there is no downward circulation, the gas can not be decomposed in the leaves, as is by many supposed. Further, we have no other evidence, than this supposition, that sunlight can decompose carbonic gas at all; nothing in my reading of chemistry would encourage the conclusion that sunlight can release oxygen from its compound. Some other agent then must be sought for, and we have one in electricity, that we know can effect this decomposition.
"The beautiful art of electrotyping is evidence of this fact Here metal is dissolved by acid, making a compound of oxygen and metal dissolved in water, by the application of galvanism, which is only another form of electricity; the oxyen is given off, and the metal is deposited in its own form again. If electricity can release oxygen from this compound, why may it not from all its compounds? Oxygen is negative, while carbon and metals are positive. This presumption has much stronger grounds for its foundation, than that for the effects chargeable to sunlight.
"Electricity has been proven to greatly facilitate vegetable growth. If we erect wires, say ten or fifteen feet high, in an open space, and pass them beneath the soil where vegetables are growing, these vegetables near and above these wires will be larger than others at a distance. Here the electricity can add nothing of itself to the plants; it must act by inducing greater activity in some principle of growth, and what principle can that be, other than the decomposition of the carbonic acid in the soil and vegetable matter within its reach, thus giving more carbonic gas to the plants? This is the only rational explanation that can be given, and it is to the point.
"Now let us apply these principles, founded upon facts. The carbonic gas being carried up in the sap, which is the water containing matters for growth imbibed by the roots, and this gas passing principally between the bark and wood, where most of the sap passes, and where it is needed most for growth, is decomposed by electricity exactly where needed, and deposited there, a small portion is deposited in the pores of the sap-wood, thus making it more firm, and converting it into heart-wood. The sap being deprived of its gas brought up from the roots, principally near the lower part of the trunk or body, is further supplied by that received by the leaves, and this passing down through the sap as it is known that this gas has a great affinity for water, would supply all parts, and will mix with it in all directions whenever it comes in contact with it, Trees and plants are as conductors of electricity from the earth to the air, and from the air to the earth. Every sprig, every point of a leaf, acts as a conductor for the fluid in passing.
Some have doubted whether there is electrical disturbance sufficient to cause action enough to produce such an effect, but electricians have proved that the electricity of the atmosphere is very variable; that it is at its maximum at one time of day, and at its minimum at another. Hence when it is at its maximum in the air, the earth must be negative to it, and the earth will receive a portion; and when at its minimum in the air, the earth will be positive, and then give off a portion to the air, thus keeping up a current between the air and the earth. Electricity having an attraction for water, will pass through the sap of plants in preference, precisely where the gas is to be decomposed.
"Here then we have a theory for the circulation of the sap, at least for the requirements of growth; one that is plain and simple, yet meets all the requirements of the case; one that is based on facts that we do know, and but little left to conjecture. At the fall of the leaf in autumn the body of the trees has but little sap; the pores of the wood are mostly filled with air. During the fall and winter, whenever the earth and air are above the freezing point, the roots are imbibing moisture. Whether this is by capillary attraction or not I leave, but presume it is by that power, as water will penetrate every body not positively impervious, where there is less water. By the time that warm weather returns in spring, this sap thus lying in the pores through the winter, undergoes some chemical change, converting part into saccharine matter. This matter thus furnishes the nectar of flowers, and is conspicuous in early flowering trees like the maples, for instance, that blossom early. Almost all trees will furnish sap at the first flow that may be made into sugar, but some, as the maples, more freely than others.
This first flow of sap takes place in the winter often, when there comes warm days and frosty nights, and frequently when the ground is frozen; but the ground being frozen is no evidence that the tree and its roots are frozen, for living plants will resist a much lower temperature without freezing than dead matter will. The roots of trees are often far below the frost, and may be supposed to be a conductor of that temperature upward. As soon as the warm weather induces a full flow of sap, the saccharine sap is diluted and carried upward, and no sugar can then be made from the body of the tree. This full flow of sap fills every pore of the tree; and as the warm weather advances causes the bud to burst and the leaves to expand, and these then become the medium of giving off the moisture of the sap, leaving the matters for growth behind. Thus carbon, gums, rosins, potash, etc., are de-posited each in its place. The philosophical editor of the 'Flores des Serres,' of Belgium, says that the offices of the leaves are chiefly perspiratory, and 'that they will so be acknowledged some day.' Many others are coming to be of this opinion.
"From the great quantity of water known to be given off from the leaves of vegetables while growing, we may reasonably conclude that matter enough for growth is carried up by the sap to account for all we see, even supposing that the amount is small in a given quantity of water. The continual rising of this matter, and being left for growth, would first make it appear as milky, as I have observed in early spring on taking off the bark of oak timber; as it becomes more dense it would assume a jelly-like appearance, as I have witnessed in taking off the bark of apple trees, about the 20th day of the first summer month, to try to cure such trees of the bitter rot. Then, again, at the fall of the leaf, it has become firm wood, and at this season there is little water in growing wood, less than at any other time. This amount of water, however, somewhat depends upon the wetness of the season. In late summer and early fall, we often have dry weather, favorable for well ripening wood, but sometimes we have wet in the fall so as to start late growth; we then know that such late wood is not able to stand the winter safely, but is often injured, particularly if cold sets in early. Late growing plants are more liable to be injured in this way, than those that perfect their growth early.
The oak and similar trees, that never start into second growth the same season, do not suffer in this way.
** We have reason to believe that the roots of perennial plants take in water as sap at all seasons, except when interrupted by drought or frost, either of which prevent it. Thus in fall and winter in mild weather, a supply is laid up for active use when the warmth of spring shall induce active growth".