of the structure of the root and stem that a beginner may still use them with advantage; such figures as those on plates 36 and 40 and elsewhere show that he knew how to fashion his observations by aid of much reflection into a clear representation of the thing seen; there are, as might be expected, many errors in the details of the more delicate structure of the various forms of vessels and cells.
Malpighi had not said, whether he considered the cells of the parenchyma (the term parenchyma comes from Grew) to be perfectly closed or porous, nor how they cohere; Grew leaves no doubt on this point; he says distinctly on page 61 that the cells or vesicles of the parenchyma are closed, that their walls are not traversed by any visible pores, so that the parenchyma may be compared to the foam of beer. He quotes Malpighi's view respecting the vessels of the wood, and supplements it by saying that the spiral band is not always single, but that two or more bands entirely separate from one another may form the wall of the vessel, and also that the spiral thread is not flat but roundish like a wire, and its turns are more or less close together according to the part of the plant. He also notices that the spiral tubes are never branched, and that when they run straight, as in Arundo Donax, they can be seen throughout considerable distances. The view of the structure of spiral vessels, which began with Malpighi and was maintained through the whole of the 18th century, Grew (p. 117) expresses still more distinctly than Malpighi; but it is to be observed that neither of them clearly distinguished true spiral vessels with separable spiral threads from vessels of the kind which occurs in secondary wood, and only shows a spiral structure on being torn. From the way, says Grew, in which the threads are woven, it comes to pass that the vessels often unroll into a flat surface, as we may imagine a narrow ribbon wound in a spiral about a round staff so that edge meets edge; and if the staff is drawn out, the ribbon so wound will remain behind
