Page:An introduction to physiological and systematical botany (1st edition).djvu/94

 64 This flowing of the sap has been thought to demonstrate a circulation, because, there being no leaves to carry it off by perspiration, it is evident that, if it were at these periods running up the sap-vessels with such velocity, it must run down again by other channels. As soon as the leaves expand, its motion is no longer to be detected. The effusion of sap from plants, when cut or wounded, is, during the greater part of the year, comparatively very small. Their secreted fluids run much more abundantly.

I conceive therefore that this flowing is nothing more than a facility in the sap to run, owing to the peculiar irritability of the vegetable body at the times above mentioned; and that it runs only when a wound is made, being naturally at rest till the leaves open, and admit of its proper and regular conveyance. Accordingly, ligatures made at this period, which show so plainly the course of the blood in an animal body, have never been found to throw any light upon the vegetable circulation. This great facility in the sap to run is the first step towards the revival of vegetation from the torpor of winter;