Grew performed a few experiments, especially in the direction of plant chemistry. This was a natural line of work for a doctor, since the extraction of various vegetable substances had long been practised in medicine. He noticed, amongst other points, that the green infusion obtained by treating a plant with olive oil would, at least in the case of certain aromatic plants, appear of a green colour in a small drop, but of a red, or deep yellow, when a quantity of it was held up against a candle. In other words, Grew seems to have observed the characteristic fluorescence of chlorophyll.
He was interested also in the subject of geotropism, and succeeded in proving that there is an innate tendency for the root to grow down and the stem to grow up; and that it is not merely a case of the root seeking the soil, and the stem the air. His directions for performing the experiment are as follows:—"Take a Box of Moulds, with a hole bored in the bottom, wide enough to admit the Stalk of a Plant, and set it upon stilts half a yard or more above ground. Then lodg in the Mould some Plant, for Example a Bean, in such sort, that the Root of the Bean standing in the Moulds may poynt upwards, the Stalk towards the ground. As the Plant grows, it will follow, that at length the Stalk will rise upward, and the Root on the contrary, arch it self downward. Which evidently shews, That it is not sufficient, that the Root hath Earth to shoot into, or that its Motion is only an Appetite of being therein lodged, which way soever that be: but that its nature is, though within the Earth already, yet to change its Position, and to move Downwards. And so likewise of the Trunk, that it rises, when a Seed sprouts, out of the Ground, not meerly because it hath an Appetite of being in the open Aer; for in this Experiment it is so already; yet now makes a new Motion upwards."
Although Grew cannot be called a great experimenter, he frequently took the easier course of throwing out suggestions for such work. "The generation of Experiments" he describes as "being like that of Discourse, where one thing introduceth an hundred more which otherwise would never have been thought of." Amongst other proposals he recommends that trial should be made of growing plants in common water, snow
