"These vessels arise in the substance of the Wood, principally towards the outer edge of each circle. They are very large in the outermost coat; and smaller in the others: and there are also irregular ranges of them, running thro' the thicknesses of the circles; besides these principal ones of the outer course. They have solid, and firm, coats; and they contain in Spring, and at Midsummer, a limpid liquor, like water, but with a slight acidity: at all other seasons of the year they appear empty, their sides only being moistened with the same acid liquor. Those who examined them at such seasons, thought them air vessels; and in that opinion, formed a construction for them, which Nature does not avow." Although Hill recognized the entity of the cell he had, in common with his contemporaries, no clear conception of its real nature. In describing the pith of the rose he does not go astray, and he fully appreciated that the seemingly double contour of the cell walls, when seen in some sections, is due to the thickness of the section with consequent overlapping of the cells; on the other hand he went very wrong in the case of the pith of the walnut, the cavities of which he supposed to be cells like those of the rose, only very much larger and uniseriate as the following quotation shews: "The Pith of the Walnut consists only of one range of these bladders ['Blebs' or cells], smaller at the edges, largest in the middle, and laid very exactly one upon the other." When he considers the structure of more or less square or oblong cells his ideas are very wrong. In such cases he thought that the transverse walls were spaces, and the longitudinal walls vessels; curiously enough Hedwig made a similar mistake some years later, possibly he was led astray by Hill's misconception. Hill adversely criticized the theory that the pith is an organ of propagation, and substituted the view that the corona--i.e. the peri-medullary zone--is all important in this connexion, "From it arises the branches, and encrease of the tree." Hill had considerable technical ability and, I think, was capable of greatly advancing anatomical botany; unfortunately, however, he gave too little time and thought to his investigations. _Physiology._ The eighteenth century saw the birth of vegetable physiology, Hales and Knight being the two great pioneers in this country. The former flourished in the early part of the century, whilst Knight, although born in 1758, published his great work in 1806. The chief physiological work of Hill is embodied in a pamphlet of 59 pages, entitled _The Sleep of Plants and Causes of Motion in the Sensitive Plant explain'd_, published in London in 1757, a year previous to the appearance of Du Hamel's _Physique des Arbres_. The paper is in the form of a letter to Linnaeus, and in it the author explains his position with regard to his earlier criticisms of the Linnaean system of classification.