In plants, as in animals, Grew looked for "vessels," and discovered by means of a simple experiment that continuous tubes, worthy of being called by this name, existed in the outer parts of the root, whereas the pith consisted of closed chambers. He cut a fresh root transversely, and then gently pressed the side of it with his finger nail. He was able to detect the vessels with the naked eye, and he observed that where they occurred, sap oozed out under pressure, but was sucked in again when the pressure was removed. The pressure also expressed a certain amount of sap from the pith, where vessels were absent, but here the sap was not sucked in again when the root was no longer squeezed, shewing that the liquid had only been forced out by the wounding of the cells. Had they been open tubes like the vessels, the release of the pressure would have caused the sap to disappear. Grew recognised that the vascular tissue of the root is centrally placed, whereas in the stem it is circumferential, and he points out that this difference is connected with the diverse mechanical needs of the two organs. It should also be noted that he discovered that concentration of the vascular system is characteristic of climbing plants, the wood, in his own words, standing "more close and round together in or near the Center, thereby making a round, and slender _Trunk_. To the end, it may be more tractable, to the power of the external _Motor_, what ever that may be: and also more secure from breaking by its winding _Motion_." He observed the radial arrangement of the xylem in the root, and offered an explanation of it, which is however scarcely free from obscurity. "Some of the more Æthereal and Subtile parts of the _Aer_, as they stream through the _Root_, it should seem, by a certain _Magnetisme_, do gradually dispose the _Aer-Vessels_, where there are any store of them, into _Rays_." Amongst other details of root anatomy, Grew discovered that 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