This section is from the book "The Gardener's Monthly And Horticulturist V23", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
In the Gardener's Monthly this subject was briefly touched, and knowing it to be an open question yet, I take the same liberty as others did before me, to give the result of my investigation. My attention to this problem was first called by Prof. Hickok, not Dr. H., as stated in your Monthly. I attended the meeting of our Society of Natural Science, the evening he read a paper on this subject, in which he demonstrated that the layers of wood were thickest on the underside of trees that sloped, and if I recollect rightly, he based his theory principally upon one large tree that grew on a steep side hill. This one he cut down and found the layers where he hoped they would be, and I believe in the Professor's eye, every tree sloped more or less. His essay was considered very nice, but there it rested. My first step was to determine why trees leaned in one or the other direction; this I soon discovered to be owing to the main growth being on that side, and this readily revealed to me that that was also the course of the main roots - even so with such as did not apparently slope. I then began to use my saw and spade, and found that on whichever side the layers were thickest, I could always locate the main root.
Now in order to avoid misunderstandings, and a disastrous collision with undeniable facts in "Plant Physiology," laid down nearly correct, by men who made this a life time study, let us see what they say about the root and its function, so as to go over the whole ground intelligently. Wood says, "the root does not absorb moisture by its whole surface indiscriminately, but only by the spongioles at the extremities of the fibrils, where the pores are not obstructed by the epidermis. From the spongioles it is conducted by the vasiform tissue of the fibril to the vessels of the main root, and immediately carried up the stem and distributed to all parts of the plant." All due respect for Prof. Wood. But in pointing out the course of the sap as it is absorbed from the earth and conducted to the stem, he is somewhat inexplicit, and apt to lead to a grave misunderstanding, he says, " from the spongioles it is conducted by the vasiform tissue of the fibril to the vessel of the main root, and immediately carried up the stem, and distributed to all parts of the plant." This would imply that all plants have but one root, to which all the others carry the sap, and that that one alone conveys it to the stem, and through some agency it is equally distributed to all parts of the plant.
But I trust that all who have read this book, were sufficiently informed, and knew that aside from a root superior in thickness, and perhaps in length, there are others which I should term subordinate, that perform the same duty, only on a reduced scale, all of which are furnished with spongioles and fibrils. It would be difficult to say definitely how many leading roots there are to every plant; but let there be many or few, among them is always one superior to all, especially in trees. This one beyond doubt carries the largest quantity of sap to the stem; as it will be seen that the branches on that side are of a more vigorous growth, and occasionally in favorable seasons a strong shoot or branch is forced out, where no one would have suspected an eye or bud to be; showing that the activity of absorption by the roots was far greater than the exhalation by the leaves. Now let us see what Asa Gray says in reference to roots and their functions. He is far more elaborate than Wood, but in substance it is nearly the same.
He says, "the fluids taken in by the roots are carried up through the stem to the leaves, and after its assimilation by the leaves, is carried down in the bark or cambium-layer and distributed throughout the plant." The word "distributed" used by these able botanists, is an ill-chosen word, and is likely to convey a wrong conception if the true meaning is taken; it carries the idea with it that it is equally distributed to each part of the tree in equal proportions. A glance at stems and branches will convince any one to the contrary. The setting off of material which is carried into the plant or tree by the roots, has its destination assigned, and goes just where it is intended it should.
Further on Gray says, "now when the proper materials are brought to the growing parts, growth takes place." Now which are the parts that do not grow when the growing season has arrived? Every part then grows, - roots, stem, branches and leaves. Both these authorities agree that the sap is carried to the tree, or plant by the roots, and that it is there converted into vegetable fabric. The question now arises how is this done? Gray says it cannot be explained by any mechanical law; but we see that it is done. Now is it distributed to all parts of the tree in equal proportions? From the looks of the stem and the different sizes of the branches we must conclude that is not, as it invariably shows a one sided growth; that is on one or the other side of the tree the growth is strongest. This proves that the main roots are on that side. Now what forces a bud to develop into a branch? And what does it prove? It shows that the activity of absorption is greater than the exhalation. Growth is an aggregation of cells; and growing is a multiplication of cells, and this is greatest where absorption is greatest.
Now if absorption outbalances exhalation to too great a degree, a new exhalator will be necessary; but before this one is ready, the stem must accommodate the new fabric, which is constantly made on that side, until the new exhalator is developed enough to do its duty.
The eccentricity of wood layers, more on one side than on the other is certainly owing to some cause, and the most plausible is, the unequal setting off of vegetable fabric; and that it is always on the side where growth is strongest, proves that the absorption there is greater than exhalation, and through this stoppage the vegetable fabric is forced to fix itself there.