This section is from the book "The Gardener's Monthly And Horticulturist V18", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
At a recent meeting of the Academy of Natural Sciences, of Philadelphia, Mr. Thomas Meehan called attention to the many varying hypotheses in regard to the eccentricity of the annual layers of wood in plants, which is sometimes so great that, as recentty shown by a writer in The American Naturalist, the pith (in the common Poison Vine), is wholly on one side, and once in a while seems like a little ridge running along just beneath the bark. In the Poison Vine the greatest thickness of wood seems generally on the side between the pith and the object the vine clings to, and the writer referred to surmises that the rootlets coming out on that side may have something to do with this interior thickening. Another gentleman, Dr. Hickok, of Poughkeepsie, thought only those trees which sloped a little thickened chiefly on the under side. These hypotheses were inconsistent, and Mr. Meehan thought the true cause of the thickening of woody layers more on one side than the other had yet to be explained. The rooting on the under side could not cause the thickening of the wood, as Wistaria and many others which he mentioned, rooting on the ground as they ran, did not thicken in consequence; while Ampelop-sis did, as well as the Poison Vine. The rootlets by which the Poison Vine attached itself to the trees had been referred to as being of some age ; but this was a mistake, as in most cases save some orchideae and a few other plants aerial rootlets, like rootlets beneath the earth, were mostly annual.
The whole root system of plants was, indeed, but the analogue of that system which existed in the atmosphere. Morphology had made a great stride when it pronounced all the parts of the inflorescence but modified leaves.
Botanical science had yet to go further. The whole plant was but a modified leaf, roots as well as branches. The same general laws that we found in the aerial system, therefore, had their correspondence in the terrestrial one. In the terrestrial system we generally saw a marked difference in the leaves and branches; but in some cases, as the arbor-vitse and deciduous cypress, the two were so blended together that at the annual "fall" season branchlets and leaves all fell together. In these cases we saw that some of these compounds of leaves and branches - those the most favorably situated as regards nutrition - maintained a hold on life, and, once passing this critical time, had an indefinite lease of life thereafter. It was precisely the same with the rootlets of trees. They were the representatives of leaves, and myriads died every year. Only here and there one endowed with greater vital privileges escaped the annual " fall," and then it became a " root," with various terms of endurance. Aerial roots, used by some creeping vines, were under the same laws. Now and then one would find itself in a soft crevice of an old wall or in the decaying hollow of an old tree, and thus become a permanent feeder to the vine.
In England the Evergreen Ivy had been cut down near the ground, after running for years over old ruins, and had continued to live on. But in these exceptional cases it was found that some of the rootlets, as the rule, annual, had found some soft place and taken on a permanent character - had become real feeding woody roots. He exhibited some old stems of Ampelopsis Virginiana, which for many years had been hanging unattached from the branches of a tree, and which had eccentric wood, as in the attached Poison Vines, and the surface was covered with aerial roots, which were produced and died annually. - Independent.