Nehemiah Grew was interested in plant physiology, although the state of chemical and physical knowledge at the time did not allow of his advancing so far in this, as in the morphological side of the subject. His turn of mind, too, appears to have naturally led him to the study of form rather than that of function. As regards the absorption of water, his idea was simply that the roots sucked up water like a sponge, because the parenchyma was of a spongy nature. He supposed that the liquid was rendered purer by being strained through the skin, which, according to whether it was of a texture resembling brown paper, cotton, or leather, would produce a different effect upon any solution passing through it. His explanation of the ascent of the sap had really much in common with the "Kletterbewegung" theory propounded by Westermaier[14] almost exactly two hundred years later. Grew argued that "considering to what height and plenty, the _Sap_ sometimes ascends; it is not intelligible, how it should thus ascend, by virtue of any one _Part_ of a _Plant_, alone; that is neither by virtue of the _Parenchyma_, nor by virtue of the _Vessels_, alone." He pointed out that the parenchyma might suck up a liquid for a short distance, and also the vessels, like "small _Glass-Pipes_ immersed in Water, will give it an ascent for some Inches; yet there is a certain _period_, according to the _bore_ of the _Pipe_, beyond which it will not rise." To account for the rise he supposes that the vessels and parenchyma work together, the turgidity of the surrounding parenchyma cells both compressing the vessels, and thus causing the liquid in them to ascend, and also actually forcing some of their own contents into them. 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.