Page:An introduction to physiological and systematical botany (1st edition).djvu/230

 200 appearances. We cannot but be aware that, when a branch is gathered, the sap must soon flow out of those spiral-coated tubes, which are large, elastic, and, no doubt, irritable. After they are emptied, air may unquestionably pass through them, especially when the whole weight of the atmosphere is acting, as in Dr Hales's experiments with the air-pump, upon so delicate a fabric as the internal vascular structure of a plant, forcing its way through pores or membranes not naturally designed to admit it. We must also recollect that a plant, cut even for a short time, begins to lose its vital principle, after which no just judgment can be formed, by any experiments, concerning the movements of its fluids in life and vigour. See Chapter 1. These experiments of Dr. Hales therefore prove no more than that the vegetable body is pervious in various directions; and perhaps the only point they correctly establish is, that air is imbibed through the bark, a part known to be full of air-vessels. But the seventh chapter of the Vegetable Staticks contains some remarks much more to our purpose. Dr. Hales there clearly anticipates