all the tissues outside the central cylinder sometimes peel off when the root becomes old, or as he says, "the whole body of the Perpendicular Roots, except the woody Fibre in the Centre, becomes the second skin." Turning to stem structure, we find that he understood the difference in origin between stem buds and adventitious roots. The stem bud, he writes, "carries along with it, some portion of every Part in the Trunk or Stalk; whereof it is a Compendium." The adventitious root, on the other hand, "always shoots forth, by making a Rupture in the Barque, which it leaves behind, and proceeds only from the inner part of the Stalk." He describes the vascular bundles of the stem as "fibres" perforated by numerous "pores." It would be a mistake, however, to suppose that he had no understanding of their structure, at least as regards the xylem, for he goes on to say that "each Fibre, though it seem to the bare eye to be but one, yet is, indeed, a great number of Fibres together; and every Pore, being not meerly a space betwixt the several parts of the Wood, but the Concave of a Fiber." He noticed the medullary rays, for which he uses the expressive term "Insertions." "These Insertions," he says, "are likewise very conspicuous in Sawing of Trees length-ways into Boards, and those plain'd, and wrought into Leaves for Tables, Wainscot, Trenchers, and the like. In all which,…there are many parts which have a greater smoothness than the rest; and are so many inserted Pieces of the Cortical Body; which being by those of the Lignous, frequently intercepted, seem to be discontinuous, although in the Trunk they are really extended, in continued Plates, throughout its Breadth."
Nehemiah Grew was interested in the process of secondary thickening, but he only arrived at a dim notion of how it took place. He grasped, however, the important point that in a tree trunk the meristematic zone lies near the surface, "the young Vessels and Parenchymous Parts" being formed annually "betwixt the Wood and Barque." He describes how, "every year, the Barque of a Tree is divided into Two Parts, and distributed two contrary ways. The outer Part falleth off towards the Skin; and at length becomes the Skin it self….The inmost portion of the Barque, is annually distributed and added to the Wood;
