Whenever Grew's notions of plant physiology depended upon chemistry, they became, according to our modern ideas, extremely difficult to follow. He held, among many other curious beliefs, that salts obtained from any plant have a tendency to crystallise out in a form resembling that plant, and adds, as an illustration from the animal world, "though I have not seen it my self, yet I have been told by one that doth not use to phancy things, that the Volatile Salt of Vipers, will figure it self into the semblance of little Vipers."
The mystical belief that characteristic "principles" permeate all things, finds expression in his idea that the "frost flowers," sometimes to be seen on a window pane, are evidence that the air is impregnated with "Vegetable Principles." Another fact, which he brings forward in support of the same view, is that the ground or water, when exposed for some time to air, turns green. His explanation, in this latter case, was not far from the truth, for, as we now know, the greenness is due to the vegetation of minute algæ, which, in their dormant state, may be carried from place to place by the wind.
It is usual to regard Ecology as a very recent development of botanical science, but Nehemiah Grew seems to have been alive to the importance of the ecological standpoint,—though he did not describe it by this name. He writes "The proper Places also of Plants, or such wherein they have ... a Spontaneous growth, should be considered. And that as to the Climate; whether in one Colder, Temperate, or more Hot. The Region; Continent, or Island. The Seat; as Sea, or Land, Watry, Boggy, or Dry; Hills, Plains, or Vallies; Open, in Woods, or under Hedges; against Walls, rooted in them, or on their Tops; and the like."
Grew's most interesting contribution to science was, perhaps, his publication of the fact that the flowering plants, like animals, shew the phenomena of sex. He never, however, actually proved this contention in an experimental way. At the time that his earliest work[15] was published, he was frankly puzzled by the stamens, or, as he calls them, the "Attire." He recognised their use to insects, to whom flowers serve, in his own words, as "their Lodging and their Dining-Room." He also fully realised their value to man as increasing the beauty of the blossom, but he was broad-minded enough to feel that these must be secondary uses, and that "the primary and private use of the attire" remained to be discovered. Ten years later, in the second edition of his work, he tells us that it was suggested to him in conversation by Sir Thomas Millington that the stamens were the male organs. It seems probable that, although Grew gives Millington the credit for this discovery, he had really arrived at it independently, for he tells us that when Millington made the suggestion, he "immediately reply'd that [he] was of the same Opinion; and gave him some reasons for it, and answered some Objections, which might oppose them."
Besides his belief in the male nature of the stamen, Nehemiah Grew came to some rather mysterious conclusions as to their serving to draw off the redundant part of the sap, not needed to produce the seed. He also used the word "attire" for the florets of the Compositæ, but qualified it by calling the stamens the "seminal attire," and the florets of compound flowers the "florid attire." He says that "every Flower with the Florid attire" (or, as we should now say, "every composite flower") "Embosomes, or is, a Posy of perfect Flowers." He recognised the "globulets" (pollen grains) as being of the same nature as those in the anthers of simple flowers. He describes the disk florets with remarkable accuracy, but falls into the error of supposing that the pollen grains are in some cases originally produced by the style and stigmas, which he calls the "Blade," and which he did not recognise as part of the female organ. His figures make it clear that he mistook the stylar hairs for little stalks organically connecting the pollen grains and the style. In other cases, however, he observed that the pollen grains occurred on the inner side of what we now know as the staminal tube.
Grew enters into considerable detail as regards the structure of flowers, and it is only possible to mention here a few of the points to which he draws attention. He observed the frequent occurrence of capitate glandular hairs, which he describes as "like so many little Mushrooms sprouting out of the Flower," their heads sometimes exuding a "Gummy or Balsamick Juyce." He describes the varieties of aestivation of the floral leaves, and notes that, in the Poppy, the large size and fewness of the petals prevents their being folded into a compact body by any of the ordinary methods. "For which reason, they are cramb'd up within the Empalement[16] by hundreds of little Wrinckles or Puckers; as if Three or Four Fine Cambrick Handcherchifs were thrust into ones Pocket."
We have said something about Grew's work on seeds, in dealing with his first treatise. He was always much interested in this subject, and returned to it again in his later work. He mentions the mucilaginous testa possessed by many seeds, but which only becomes noticeable when they have been moistened. That of "Nasturtium Hortense" he describes as very large, "even emulous of the inner Pulp surrounding a Gooseberry-Seed." He suggests that the value of putting a Clary seed into the eye to bring out a foreign body, which may have lodged there, is due to the presence of the mucilaginous coat. The same seed is still, I believe, used for this purpose, under the name of "eye seed." Grew understood the difference between seeds with, and without endosperm, and gives perfectly clear representations of such albuminous seeds as Ricinus. He describes the cotyledons of the Dock as being immersed in the endosperm, "as in a Tub of Meal or a little pot of pure refin'd Mould, necessary for the first Vegetation of the Radicle."
Grew naturally reckoned the spores of Ferns among seeds. The seed-case of the Harts-tongue is, he says, "of a Silver Colour ... of a spherick Figure, and girded about with a sturdy Tendon or Spring, of the Colour of Gold: ... So soon as ... this Spring is become stark enough, it suddenly breaks the Case into two halfs, like two little cups, and so flings the Seed," of which "ten Thousand are not so big as a white Pepper Corn."
To give any kind of short summary of Grew's botanical work is well-nigh impossible. Some men are remembered for individual discoveries, and in such cases it is not difficult to give a précis of their contributions. But Nehemiah Grew is remembered because, contemporaneously with Malpighi, he actually created the science of plant anatomy,—a subject which, before his day, was practically non-existent. Modern botanists, conscious how small an addition to the fabric is now regarded as a satisfactory life-work, must stand amazed and somewhat humbled before the broad and sound foundations laid by this seventeenth century physician. It is no less than two hundred and forty years since Grew sent in his first treatise to the Royal Society, so it is scarcely wonderful that a number of his results have been rejected in course of time. It is far more remarkable that so many of his conclusions—and those the more essential ones—have been merely confirmed and extended by later work. Great however as were his actual contributions to botanical knowledge, they were perhaps less important than the far-reaching service which he rendered in helping to free biological thought from the cramping belief that the one and only object of the existence of the organic world was for the use and pleasure of man. Grew believed that the "Outward Elegancies of Plants" might be for the purpose of giving delight to the human race, but he was the first to point out that as the "Inward Ones, which, generally, are as Precise and Various as the Outward," are so seldom seen, their purpose can hardly be for this, but must be for the benefit of the plants themselves, "That the Corn might grow, so; and the Flower, so, whether or no Men had a mind, leisure, or ability, to understand how."