Pruning, Branching and “Polarity”
IT is a disappointing thing when one is looking for a satisfactory explanation of any particular phenomenon to be told that it is due to some internal force which cannot be defined except by its results. When the candidate for a medical degree was asked why opium acted as a soporific, he is reported to have answered, "Quia est in eo virtus dormitiva." Unfortunately we have in many cases no better answer to give about the commonest phenomena. We talk glibly enough about gravitation, we assign particular effects to it as a cause, but we are as entirely ignorant of what gravitation is as we are of what constitutes heat or what is the nature of electricity. We allude to the subject because a Swiss Professor has lately been investigating the upward growth of the stem and the downward growth of the root, but after all his experiments all he can tell us is that it is due to "polarity," and this polarity manifests itself in the fact that branches tend to produce new branches near their summit, while in roots the tendency is to produce branches near the base.
M. VÖCHTING is at great pains to show that this polarity manifests itself in detached portions of the plant, as when a branch of a Willow is suspended in a dark room in a reversed position. After a while, buds will be formed at the organic upper end and roots at the upper extremity in spite of their reversed position. We need not specify M. VÖCHTING'S experiments, because they are just what experience has taught us to expect, and what many of us have frequently witnessed.
It is, however, important to remark that M. VÖCHTING proves that gravitation has nothing to do with the phenomena, for on so repeating the experiments (by the use of a clinostat) that the action of gravity is nullified, the results remain the same. Alluding to the different kinds of branches that one meets with in fruit trees, M. VÖCHTING shows how each variety of branch, leader, shoot, lateral, fruit-spur, spine, may proceed from buds of identically the same character and that the character of the shoot may be determined by the experimenter. It is obvious here that we are dealing with the physiological basis of pruning. The facts specified by M. VÖCHTING have been known and acted on for centuries, and various explanations have been given of them. In gardening books, even many of the best modern books, we find the sap credited with this, that, and the other, according to its nature, and the supposed directions and concentration of its currents.
The teaching of modern physiology takes a very long time filtering through horticultural books, and so we find even now the best gardeners among us still hugging the belief in the mysterious and diversified powers of the sap, long after such powers have been shown to be purely visionary. We need not now stop to point out what the accepted doctrine is, as we have so frequently done so in these columns, and it is given in all the modern botanical text-books. That no attribute of the sap, so far as we know at present, suffices to explain polarity in M. VÖCHTING'S sense, is made by him abundantly obvious, but unluckily, while like his predecessors among vegetable physiologists he takes away our sap as a motive force, he does not provide us with any more tangible substitute than an "internal force" called polarity. It is something—it is a great deal, however, to have errors eliminated—destructive criticism at least paves the way for constructive action; and so we may in passing allude to some peculiarities pointed out by M. VÖCHTING which are not generally known. Thus, if a detached branch be suspended in a horizontal or sloping position buds will, under suitable conditions, be formed all round the summit and on the undersurface near the summit, while the roots will spring at the base from the circumference of the branch and from the lower surface of the branch for a certain distance from the base. In the branch still attached to the tree the buds at the apex of the shoot forming the crown are always developed, the other buds on the upper side of the shoot are developed in a degree corresponding with the more or less horizontal position of the branch. In the case of the pendulous branches (as of the Weeping Ash for example) when severed from the tree, the crown of buds at the end is produced as usual, while the other buds are equally developed on all sides. As to the subsequent growth of these buds the author shows that the force of growth is greater in proportion as the shoots have a vertical direction and the force of growth is universally proportionate to the angle which the branches make with the trunk.
In the case of branches, which are directed more or less horizontally, the force and direction of growth depend upon the distance of the bud from the apex of the shoot, the degree of development already obtained, the degree of inclination of the shoot, and its position on the upper or lower surface of the branch, as the case may be. In the Pear and the Apple the diminished intensity of growth on the lower surface of the shoots naturally favours the development of flowers.
The author sums up the general results of his experiments thus:—"In considering the mode and direction of branching two factors have to be considered—an internal force—'polarity'—and gravitation. Polarity tends to the production of shoots from the tip or free end of the branch; gravitation, on the other hand, induces the development of branches in the highest parts. According to inclination to the horizon, these factors may act together or in opposition one to the other. In other words gravitation joins with polarity when the position of the branch is between horizontal and vertical (less than 90°), and it hinders polarity when the inclination is more than 90°, and it directly opposes it when the direction of the branch is vertically downwards."
As to the relation between roots and branches M. VÖCHTING relies upon such facts as these:—An Apple tree growing on the border between turf on the one side and a kitchen garden on the other will develope itself more fully on the side towards the tilled ground. If a strong root of a fruit tree be severed one of the branches suffers an arrest of growth; and other considerations are adduced to show the relation between roots and leaf-bearing branches.
Beginning with circumstances of this character M. VÖCHTING next directs his attention to practical matters, such as the culture of fruit trees in pots, root-pruning, and ringing. In the case of pot fruit trees, where the roots are confined in a restricted space, the whole of the vegetative system is checked; each branch produces but a single shoot, and cultivators are sometimes under the necessity of "heading back" to secure a sufficiency of branches. On the other hand the production of flowers is favoured to a great degree. The life of the tree is, however, shortened by these procedures. Root-pruning rests on a similar basis: if the strong roots are severed occasionally the tree produces a mass of fibrous roots; the production of wood is much restricted, that of flowers is promoted. Lifting and transplanting have the same effect.
Ringing has the same effect as a total severance of the branch, and is equivalent to the formation of a new individual, which grows independently of the mother plant, receiving from it merely water. The portion of the plant above the wound then behaves like a plant whose roots have been cut. The production of wood is checked, but that of fruit is favoured. Above the wound from the base of the new individual plant, if it may be so called, roots are produced, while from the upper end of the parent branch below the wound spring branches, and thus the "polarity" above alluded to is once more called into play. These facts are familiar enough to all gardeners, so much so, that they will be as much surprised to see them treated at such length in a physiological treatise as the physiologists are astonished at the wonderful theories invented by the gardeners to explain their procedure. But after all, the reference of the phenomena to intangible forces, external or internal, is not much more satisfactory than the allegations as to the ebb and flow of the sap, except to those to whom a sounding phrase, like "that blessed word, Mesapotamia," conveys ample satisfaction.