This section is from the book "The Gardener's Monthly And Horticulturist V22", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
Before setting out on the little jaunt up the Brandywine creek, I came upon a Paper Mulberry overhanging a fence at the Corner of 13th and King streets. This mulberry, like one which I had previously described in the Gardeners' Monthly, had sent out roots from the new wood formed along an opening in the tree's side. The opening in this latter case had doubtless once been completely filled with brown or black decayed wood, but a portion of this having been knocked or weather-washed away, one or two large roots starting from a point about eight feet above the ground were brought into view. These roots were an inch and a half in thickness.
In the published description of a similar case above mentioned, I expressed an uncertainty as to whether the roots and rootlets filling the decayed wood, were from the entire inner surface surrounding said decayed wood, or only from the new wood on the margins of the opening. Tree number two made this point clear, - the roots were the product of the new wood only. A second break in the side of this tree, where a large branch diverged from the trunk seemingly caused by insufficient support below, gave rise to another peculiar formation. Here the marginal new wood crossed from one side of the break to the other, forming a sort of Siamese-twins' union of the two sides. Above this cord of union, and quite within the break, a stout twig had grown from a surface of new wood which was beyond the root-developing action of of the black, heat-absorbing decayed wood. This tree also furnished an instance of the new wood continuing to grow laterally until it formed a circle in section, and breaking away on both sides, became as far below as its junction with the main trunk, a self-supporting lesser trunk.
Query: Why is this tendency of rooting from the new wood a peculiarity of Broussonetia papyrifera? My supposition is, that it is the complete decay that occurs within, together with the dark, almost black color of this decayed wood which gives it thus the stimulating properties of rich earth.