In what way are new layers of wood added to the stents of growing exogenous trees'!

This is a mooted question among vegetable physiologists, and as the subject has been brought forcibly to my mind, by an example which I have met in pruning, I thought it might not be unprofitable to draw attention to this very interesting subject.

The popular theory, I believe, is that the moisture of the soil enters the roots of plants, through the spongioses or porous points, by absorption - that there it combines with soluble matters already in the roots and stem, and becomes what is called sap - that this ascends through the cells or organs of the stem, into the leaves, where, by parting with water by exhalation, and receiving carbonic acid gas from the atmosphere by absorption, it undergoes certain changes, becomes duly elaborated and fitted for the formation of new parts, when it takes the name of cambium. Then this cambium or elaborated sap, passes downwards, through the inner bark, and depositee a new layer of wood on the top of the previous one, and a new layer of bark within the previous one. - and thus what we call the concentric layers or annual rings of wood, by which we count the ages of trees, are formed.

This, as I have remarked, is the most popular theory, at least so I regard it, of the formation of exogenous wood. But there are some learencd physiologists and botanists who dissent from this theory.

Dr. Schleiden, extraordinary professor of Botany in the University of Jena, in his "Principle of Scientific Botany," a learned and valuable work, says, " this is only a and at whatever time, we examine the sap of a plant, we find that it contains organic principles which cannot come from the soil, because they do not exist there; such as starch; sugar, gum, malic, citric and tartaric acids, albumen, etc. These substances are diluted with a good deal of water, and mixed with a little carbonic acid and carbonate of ammonia which are contained in the water of the soil. Even in the cells of the roots which first receive the moisture of the soil, it is chemically changed, assimilated, and the sap is most decidedly not flowing in special vessels but-passing upwards from cell to cell, and thus it is in every new cell which is being developed by the formative chemical processes. Nothing remains for the leaves to assimilate."

This you see is an utter denial of the ascent of the sap in special vessels, of its elaboration in the leaves, or descent in the bark. Now if he be right, how is it possible, that, when the ascent of the sap is obstructed by the compression of the stem as with a ligature the upper part, which is less in contact with the sap than the lower part, can increase much more rapidly in size? The example to which I have referred is that of the branch of a plum tree encircled by the wire of a label. This branch is four years old, and during the whole of last season and part of the one previous, this wire has been so tight that no enlargement of the wood under it could take place, the consequence is that the part just above the wire is one inch greater in circumference than that below it, so much greater have been the deposites of new matter above than below. Besides it happened that on one side the wire did not press so firmly, nor so soon as on the other, and on that 6ide of the part below the wire, we find the last layer of wood three times as thick as on the other side, where the pressure was first and greatest, and the separation more complete.

The annexed drawing represents the branch referred to, A the large portion above the wire, and B the smaller portion below. C, D, the point encircled by the wire. On the side D, the wire admitted of greater expansion, and there the upper and lower lip of the wound project almost equally. On the side C, the upper lip is a regular perpendicular wall 3-16 of an inch deep, the surface of the lower part being quite even. On the cut section E, we find the last ring of wood on the side A, D, B, 3 times as large as on the other, on account of the partial communication existing at the point D.

Does not this furnish a pretty strong indication that the formation of new layers of wood, is a downward process, and that it depends upon the leaves.

Dr. Schleiden accounts for such cases by saying, "As water is continually exhaled by plants in proportion to the motion, dryness and warmth of the air, so the sap becomes concentrated, and thus interrupts the endosmotic process towards the other cells; this action is continued naturally downwards towards the roots, by which new watery and un-assimilated fluids are absorbed. If this stream of crude sap is artificially interrupted in its course from below upwards, the sap in the upper part becomes more concentrated, and its organizing power increased. This is the simple fact which lies at the foundation of all the phenomena which are brought forward to support the groundless hypothesis of a descending bark sap. The two most important facts upon this subject are: 1. The magic ring (ringing fruit trees.) 2. The action of grafts.

"If from the circumference of a branch or tree, a ring of bark be removed, the upper part will bear richer blossoms and fruit; the latter will ripen quicker, the leaves will be thrown off sooner, and the trunk will become thicker and stronger than in the part below the cutting. All this is completely explained in the foregoing facts, without making it in the least degree necessary to assume the motion of any descending proper juice or bark sap which certainly does not exist."

Someremarks On Vegetable Physiology 60031

"When an apricot graft grows from the trunk of a plum tree, the latter is naturally and by degrees clothed with apricot wood, for out of the same soil, an apricot tree would merely take up the same sap as the plum tree; but afterwards, in proportion as the leaves and branches of the plum tree, or of the apricot, evaporate, assimilate, Ac., plum or apricot wood will remain."Such is Dr. Schleiden's mode of reasoning, and really the case of the graft presents at first sight the strongest objection to the descending sap theory, because if the new woody matter is prepared in the leaves and is deposited by a downward process, is it not reasonable to suppose that when the pear grows on a quince stock, new layers of pear wood will be deposited on the quince? But no such phenomenon occurs, and as I believe for this reason. The cambium or elaborated sap, is only a prepared condition of the food of trees, and when it happens, as in the case of the pear on the quince, that one species prepares cambium for itself and another, each one receives it as food only and appropriates it to its own peculiar formation in the way that two species of plants will grow in precisely the same soil without losing their identity, or two species of animals subsist on the same food without assuming any degree of similarity.

I do not doubt, nor is it denied, that I am aware of, by any physiologists that the sap undergoes a certain degree of elaboration in the cells and organs of the stem, but that it is principally performed by the leaves, and that the principal part of the new wood is formed by the descending prepared sap, seems most in harmony with the facts that daily occur to us in practice.

P. B

Rochester, N. Y., April, 1851.