Grew's clearest account of plant cells is perhaps to be found in his description of root parenchyma, which he compares to "the Froth of Beer or Eggs" or to "a fine piece of Manchet[11]," or again, to "a most curious and exquisitely fine-wrought Sponge." He quotes with approval Hooke's description of Elder-Pith as "an heap of Bubbles." It would be unsafe however to conclude that he had really arrived at what is known as the Cell Theory. His conception of the nature of plant tissues was not by any means that of the modern botanist. He believed the cell-walls to consist of inter-woven fibres, which were continuous from cell to cell. He did not consider that these fibres were invariably wrought together in such a fashion as to enclose bladder-like spaces, or cells; in some cases he held that the tissue was non-cellular, consisting simply of interwoven fibres. It was these hypothetical fibres, rather than the cells, which he regarded as of fundamental importance. His idea, which is somewhat confusing, is perhaps best understood from his comparison of plant structure with pillow lace. The "most unfeigned and proper resemblance we can," he writes, "at present make of the whole Body of a Plant, is, To a piece of fine Bone-Lace, when the Women are working it upon the Cushion, For the Pith, Insertions[12], and Parenchyma of the Barque, are all extream Fine and Perfect Lace-Work: the Fibres of the Pith running Horizontally, as do the Threds in a Piece of Lace; and bounding the several Bladders of the Pith and Barque, as the Threds do the several Holes of the Lace; and making up the Insertions without Bladders, or with very small ones, as the same Threds likewise do the close Parts of the Lace, which they call the Cloth-Work. And lastly, both the Lignous and Aer-Vessels, stand all Perpendicular, and so cross to the Horizontal Fibres of all the said Parenchymous Parts; even as in a Piece of Lace upon the Cushion, the Pins do to the Threds. The Pins being also conceived to be Tubular, and prolonged to any length; and the same Lace-Work to be wrought many Thousands of times over and over again, to any thickness or hight, according to the hight of any Plant. And this is the true Texture of a Plant."
Grew thus visualised the inner structure of the plant as a textile fabric, and the analogy between vegetable substance and woven threads seems to have been constantly present in his mind. The same idea also occurs, for instance, in the dedication of his magnum opus, where he says, "one who walks about with the meanest Stick, holds a Piece of Nature's Handicraft, which far surpasses the most elaborate Woof or Needle-Work in the World."
The notions at which Nehemiah Grew arrived on the subject of the vascular anatomy of plants were more advanced than his ideas on the ultimate nature of the tissues. There is no doubt that the comparison with animal anatomy, which was constantly in his mind, was on the whole helpful, though it led to some errors. The following paragraph, which occurs in the Cosmologia Sacra, seems to be an instance in which the analogy with the animal kingdom, helped him to take a broad view. "In the Woody Parts of Plants, which are their Bones; the Principles are so compounded, as to make them Flexible without Joynts, and also Elastick. That so their Roots may yield to Stones, and their Trunks to the Wind, or other force, with a power of Restitution. Whereas the Bones of Animals, being joynted, are made Inflexible."
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 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; the Parenchymous Part thereof making a new addition to the Insertions within the Wood; and the Lymphæducts a new addition to the Lignous pieces betwixt which the Insertions stand. So that a Ring of Lymphæducts in the Barque this year, will be a Ring of Wood the next; and so another Ring of Lymphæducts, and of Wood, successively, from year to year." Exactly what Grew meant by the term "Lymphæduct" is not always clear. In some cases he seems to refer to the phloem and cambium by this name, and in other cases to the perimedullary zone. The annual rings in Oak, Elm, Ash, etc. came under his observation, and he remarks that the difference between the Spring and Autumn wood, as we should now call it, arises from the fact that "the Aer-Vessels that stand in the inner margin of each annual Ring, are all vastly bigger, than any of those that stand in the outer part of the Ring."
Plate VII
From Grew's Anatomy
Sheweth the Parts of a Goosberry