But it is true that the conditions in which a bud grows render it liable to extrinsic ills not incidental to a plant springing from seed. A seed, emitting its roots directly into the earth, is liable only to its own ills; a bud or graft emitting roots, through the alburnum of the stock on which it is established, into the earth, is subject to the infirmities of the stock as well as to its own. Thus a healthy seed produces a healthy plant. A healthy bud may produce a feeble plant, because inoculated upon a diseased branch or stem.
Instead of a limitation in their nature, there is reason to suppose that trees might flourish to an indefinite age were it not for extrinsic difficulties. A tree, unlike an animal, is not a single, simple organization, it is rather a community of plants. Every bud separately is an elementary plant, capable, if disjoined from the branch, of becoming a tree by itself. In fact, each bud emits roots, which, uniting together, go down upon a common support (the trunk) and enter the earth, and are there put in connection with appropriate food. Every fibre of root may be traced upward to its bud from which it issued.
In process of time, the elongation of the trunk exposes it to accidents; the branches are subject to the force of storms; in proportion as the distance from the roots increases, and the longer the passages through which the upper sap, or downward elaborated sap travels, the more liabilities are there to stoppage and injury. The reason of decline in a tree is not to be looked for in any exhaustion of vital force in the organization itself, but it is to be found in the immense surface and substance exposed to the wear and tear of the elements.
It would seem, if this view be true, that no bounds can be placed to the duration of perennial plants, if, by any means, we could diminish their exposure, by reducing their expansion, by keeping them within a certain sphere of growth. Now this is exactly what is accomplished by budding.
A bud, far removed on the parent stock from the root and connected with it through a long trunk, is inoculated upon a new stock. It now grows with a comparatively limited exposure to interruption or accident. The connection with the soil is short and direct.
In this manner a variety of fruit may be perpetuated to all generations, if the laws of vegetable health be regarded in the process. Healthy buds, worked upon healthy stocks and planted in wholesome soil, will make healthy trees; and from these another generation may proceed, and from these another. By a due regard to vegetable physiology, the Newtown Pippin, and the Seckle Pear, may be eaten two thousand years hence, provided, always, that expounders of prophesy will allow us the use of the earth so long for orchard purposes. A disregard of the laws of vegetable physiology in the propagation of varieties, will, on the other hand, rapidly deteriorate the most healthy sort. There is no clock-work in the branches of the tree, which finally runs down past all winding up; there is no fixed quantity of vitality, which a variety at length uses up, as a garrison does its bread. Plants renew themselves and every year have a fresh life, and, in this respect, they differ essentially from all forms of animal existence. Any one tree may wear out; but a variety, never.
We need not say, therefore, that we dissent from Knight’s theory of natural exhaustion and from every supplement to it put forth since his day. Van Mons’ theory of variation and the tendency of plants to return toward their original type, is to be regarded as nearer the truth.