Somewhat approximating to the forementioned phenomenon, is the “rotation” of fluid in the cells, or, as it is usually termed, the _circulation of the sap_, of plants. This is shown in certain vegetable growths as a constant stream of thick fluid, wherein small globules are seen; which stream flows round the individual cells, or up the leaf, turning at the extremity, and down again by a different but parallel channel. There is little or no difficulty in showing this in many plants; but some are, of course, better than others, and require a different treatment; we will, therefore, notice a few of these. Perhaps the best of all is the _Vallisneria spiralis_, which is an aquatic plant, frequently grown in, but not really belonging to, this country. As it somewhat resembles grass, the leaf is not used in its natural state, but a thin section cut lengthwise with a razor or other sharp instrument--this section, however, is much better when the outer surface has been first removed. It should then be laid upon a slide with a drop or two of water, and covered with a piece of thin glass. Often the cutting of the section seems to be such a shock to the leaf that no motion is visible for awhile, but in a short time the warmth of an ordinary sitting-room will revive it, and with a quarter-inch object-glass the currents will be rendered beautifully distinct. Where the “stream” is unusually obstinate the warmth may be slightly increased, but too great heat destroys the movement altogether. In the summer, any of the leaves show this “circulation” very well; but in the winter, the slightly yellow ones are said to be the best.