Popular Science Monthly/Volume 42/November 1892/Economical Trees
By FREDERICK LE ROY SARGENT.
THE well-known power which many plants possess of developing adventitious roots from almost any part, when placed under favoring conditions, is manifested in a somewhat extraordinary manner by several trees recently brought to the notice of botanists.
In the Bulletin of the Torrey Botanical Club for August, 1891, the present writer published an account of a linden growing in Boston, Mass., where it had been subjected to injury from horses gnawing the bark, and in consequence had a considerable portion of the trunk decayed, as shown in the accompanying sketch (Fig. 1). At the edge of the wound the cambium had formed a callus, and from a point in this living tissue there proceeded several vigorous roots which penetrated the decaying wood in all directions, evidently finding a rich soil.
Subsequent issues of the Bulletin have contained descriptions of several other examples of trees exhibiting a similarly economical utilization of the products of their own decay. These include swamp maples, a Norway maple, a willow, and a white mulberry. In an English paper appeared not long ago an account of an oak which had "sustained itself for years by a mass of roots grown into its own trunk!"
In one of the swamp maples observed by L. M. Stabler, at Great Neck, Long Island, the primary injury apparently resulted from a storm which split and twisted the trunk. One of the adventitious roots, "at least two inches in diameter, started as high as ten feet above the base of the trunk, and passed down through the decayed portion to the ground" (Fig. 2).
The Norway maple, described by W. A. Buckhout, had "a large branch split off, showing that the splitting had started several years before, that the margins of the trunk had become well calloused, and from several points roots had extended into the cleft, which naturally became partially filled with dust and decaying bark. The largest root was an inch in diameter, divided considerably near the lower end, and was over two feet long."
Of the small white mulberry growing on the grounds of the Department of Agriculture at Washington, Mr. Sudworth says: "The conditions are essentially the same as those noted in the case of the linden, except that the mulberry is perhaps more seriously injured, a considerable portion of the trunk having been destroyed by decay. The adventitious roots observed spring from the free border of a longitudinal crack where the trunk forks, the edges of the wound having been healed for some time, while the subsequent decomposition of the inner layers of wood formed a quantity of mold, which, lying in contact with the
healed borders, seems to have induced the growth of adventitious roots from one side into the decayed mass."
To this list may now be added another mulberry (Fig. 3) observed by the writer during the past winter in Thomasville, Ga. Its owner, Dr. T. S. Hopkins, says of it: "I have had an intimate acquaintance with this grand old tree for thirty years. I do not know how old it was when I first knew it. Some fifteen years ago it was uprooted by a storm. I carefully amputated its limbs and re-erected its body. It lived and improved, and to-day furnishes as much shade as it did before its fall and the surgical operation made necessary by it." In point of size, extent of decay, and the number and thickness of its adventitious roots it would seem to be much the most striking example of an economical tree thus far described. The trunk is now about three feet or more in diameter, and so much decayed as to leave merely a shell of
Fig. 3.—Trunk of Mulberry, growing in Thomasville, Ga. (Sketched by the writer.)
no great thickness. The adventitious roots are some of them as thick as a man's arm. They all ramify through the disintegrating heart of the tree, and the longest of them appear to reach the earth. Besides saving from waste the products of decay, these roots must add considerable strength to the weakened trunk. This feature is perhaps all the more significant in view of the mulberry's near kinship with the banyan tree, which makes such wonderful mechanical use of aërial roots.
With regard to the way in which such economizing roots originate, and their physiological significance, it seems clear, as Mr. Sudworth has suggested, that the conditions necessary for their production are essentially the same as those favoring root-production in cuttings and layered branches. That is to say, given a vigorous cambium or similar formative tissue, near a more or less injured region, the presence of moisture for a certain period, and a congenial soil, then adventitious roots may be expected to appear. That in all the cases above cited these conditions were most probably present antecedent to the appearance of the roots seems surely to be a fair inference from all we know regarding them.
When, as in Mr. Buckhout's maple, there is opportunity for dust, etc., to accumulate in a small cleft near the callus, before total separation of the limb, the conditions are practically the same as in those not uncommon cases where seeds are found to sprout in the fork of a tree and grow for a number of years.
Now that the attention of observers has been called to this curious power which trees have of making the best of a bad matter, it will doubtless be found that the phenomenon is of more common occurrence than was at first suspected.