The most important part of this treatise is the account of the comparative structure of roots, to which we will return later, when discussing Grew's anatomical conceptions. With regard to the position of the plant in the soil, he held somewhat mystical views. He believed that the "air-vessels" or tracheal elements, tended to draw the plant upwards, and the roots to pull it downwards. He says, for example, that the upper part of the roots of most seedlings ascend, because the first leaves being large and standing in the open air, "the Air-vessels in them have a dominion over the young Root, and so yielding themselves to the sollicitation of the Air upwards, draw the Root in part after them."
In 1675 appeared Grew's third botanical work, The Comparative Anatomy of Trunks, which dealt with stem structure, as the previous work dealt with root structure. There is, in the British Museum, a particularly interesting copy of this book, which is elaborately annotated in manuscript. From internal evidence it seems almost certain that this is the author's copy, corrected in his own handwriting[10]. Some, though not by any means all, of the corrections are identical with the alterations found in the 1682 edition. Above the first plate is written "vide ye Book Interleavd," and we may perhaps hazard the guess that in this copy we have Grew's first suggestions, whilst those which he finally adopted in the second edition were inserted in the interleaved copy whose whereabouts, if it still exists, is unknown at the present time.
[Pl. 6] shews a typical page from the annotated copy. At the foot we find the note "Air-Vessels out of Parenchyma, transformed, as Caterpillars to Flys," shewing that Grew had arrived at some idea of the formation of vessels. The whole section of the book to which this page belongs is very much remodelled in the 1682 edition, but the analogy just quoted is introduced and Grew proceeds accurately to describe the origin of vessels. "And as the Pith it self, by the Rupture and Shrinking up of several Rows of Bladders, doth oftentimes become Tubulary: So is it also probable, that in the other Parenchymous Parts, one single Row or File of Bladders evenly and perpendicularly piled; may sometimes, by the shrinking up of their Horizontal Fibres, all regularly breakone [sic] into another and so make one continued Cavity."
I have passed over these three treatises in a somewhat cursory fashion, because Nehemiah Grew's botanical work is perhaps better studied in his final pronouncement on the subject,—a folio volume published in 1682 under the title of "The Anatomy of Plants. With an Idea of a Philosophical History of Plants. And several other Lectures, Read before the Royal Society." This work consists of second editions of his three earlier treatises, largely rewritten, with a great deal of additional matter, including a section on the anatomy of flowers, and many new figures. Some of the plates are excellent, and especially remarkable for the way in which Grew shews the anatomy in drawings which represent the organ in three dimensions ([Pl. 7]). He himself laid great stress on this. In his own words, "In the Plates, for the clearer conception of the Part described, I have represented it, generally, as entire, as its being magnified to some good degree, would bear.... So, for instance, not the Barque, Wood, or Pith of a Root or Tree, by it self; but at least, some portion of all three together: Whereby, both their Texture, and also their Relation one to another, and the Fabrick of the whole, may be observed at one View." One cannot help wishing that botanists of the present day would more often take the trouble to illustrate their papers on this principle.
Plate VI
A page from The Comparative Anatomy of Trunks, Nehemiah Grew, 1675. The annotations are believed to be in the author's own handwriting. [British Museum. Printed Books Dept. (972.a.10)]
It is as a plant anatomist that Grew is chiefly famous, and it is important to try to realise exactly how far his conception of the anatomical structure of plants has been confirmed by more recent research. In appraising his work it must be remembered that he was essentially the pioneer of the science. It is true that some observations on plant anatomy occur in Robert Hooke's Micrographia, which was published six years before Grew sent in his first manuscript to the Royal Society; but Hooke never really attempted to make a systematic study of the subject. He had succeeded in greatly improving the microscope, and his chief interest was in applying his instrument to all kinds of bodies, vegetable and otherwise. Cork, charcoal, pith, etc., came under his observation, and to some extent he understood their structure. Grew acknowledges indebtedness to "the Learned and most Ingenious Naturalist M r Hook," and tells us that some of the results which Hooke obtained, inspired him to study certain of his plants again with a better microscope. For instance Hooke was able to see smaller pores in wood than Grew had been able to detect, but, with better glasses, he confirmed the accuracy of Hooke's observation. However, although Hooke must certainly be credited with priority in the discovery of the fact that plant tissues are characterised by a cellular structure, his botanical work, considered in its entirety, is of very slight significance compared with that of Grew.