THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS types he saw and figured for the first time (he was the first to use the term parenchyma for instance) he gave early attention to the walls of vessels. On mechanical treatment of these he found them to unwind like flat ribbons and of these ribbons he says (1682) (Plate I): ... the Vessles, oftentimes, unroave in the form of a Plate. As if we should imagine a piece of fine narrow Ribband, to be woutf d spirally, and Edg to Edg, round about a Stick; and so, the Stick being drawn out, the Ribband to be left in the Figure of a Tube, answerable to an Aer-Vessel. For that which, upon the unroaving of the Vessel, seems to be a Plate, or one single Piece, is, as it were, a Natural Ribband, consistmg of several pieces, that is, a certain number of Threds or Round Fibres, standing parallel, as the Threds do in an Artificial Ribband. And as in a Ribband, so here, the Fibres which make the Warp, and which are Spirally continu'd; although they run parallel, yet are not coalescent; but contained together, by other Transverse Fibres in the place of a Woof. He became convinced that all other cell types follow this structure, and concluded in general that the threads in parenchyma cell walls lie horizontally while those of fibres lie vertically. Let us notice particularly the fineness to which he considered these threads could be split. So in the Pith of a Bulrush of the Common Thistle, and some other Plants; not only the threds of which the Bladders; but also the single Fibres, of which the Threds are composed; may sometimes with the help of a good Glass, be distinctly seen. Yet one of these Fibres, may reasonably be computed to be a Thousand times smaller than an Horse-Hair. This latter estimate can hardly be accepted since the fibres would then have been ofthe order of 0-1 or 0-2/i in diameter. It does seem reasonable to assume, however, that Grew had seen threads grading in fineness down to the limits observable. Some two hundred years later Sachs, the foremost plant physiologist of his time, was to ridicule these suggestions made by Grew; yet it is very instructive to examine his figures, of which one is presented in Plate I, in the light of modern observations with an electron microscope (Frontispiece). It is always easy, of course, to read modern ideas into older and vaguer writings, but here Grew expresses himself so unequivocally that one can hardly avoid the conclusion that this interpretation, in terms of what we would now call fibrils, was inspired vision. We are to see the idea of fibrillar structure turning up again and again in the years that followed, both in the wall and in the protoplasm, and equally often being ridiculed. Grew was undoubtedly an acute observer; among other things, he realized that the walls of parenchyma cells were complete, without perforations, a point structures was any attempt made at an understanding of submicroscopic features or therefore at an impression of the processes of growth which such knowledge alone can give. This was, however, soon ...
