Hales is the last of the great naturalists who laid the
foundations of vegetable physiology. Strange as some of their
ideas may seem to us, yet these observers were the first who
gained any deep insight into the hidden machinery of vegetable
life, and handed down to us a knowledge both of individual facts
and of their most important relations. If we compare what
was known before Malpighi's time with the contents of Hales'
book, we shall be astonished at the rapid advance made in less
than sixty years, while scarcely anything had been contributed to the subject in the period between Aristotle and Malpighi.
3. Fruitless attempts to explaiA the movement of the sap in plants. 1730-1780.
If those, who studied the nutrition of plants and especially the movement of their sap in the period between Hales and Ingen-Houss, had kept a firm hold on Malpighi's view, that the nutritive substances are elaborated in the leaves, and had combined it with Hales' idea, that plants derive a large portion of their substance from the air, they would have had a principle to guide them in their investigations into the movement of the sap; and by experimenting on living plants they might have succeeded in giving a more definite expression to these ideas, even though chemistry and physics supplied during that time no new aids. We have said already that such was not the course of events; physiologists confined their attention to the obvious phenomena of vegetation, and trusted in so doing to gain a firmer footing, but in this they never got beyond a commonplace and unreflecting empiricism, because their observation was without an object, and their conclusions without a principle. They wandered from the right direction, as always happens when observation is not guided by a well-considered hypothesis; and their conceptions were rendered more obscure by their imperfect acquaintance with one of the most important aids to understanding the movement of the
