One cannot help being struck with the fact that green is the prevailing colour among plants and the reason is not far to seek. If we take a cabbage leaf and carefully tear off the skin, we shall find green spongy matter below. A little of this green material may be examined under the microscope and will show us rounded green bodies composed of a substance called chlorophyll. Now chlorophyll is absolutely necessary to all plants, except the fungi and to one or two parasitic plants. It is necessary because, by its aid, plants can build up raw food material into food which will be useful to them. It is not formed in darkness; that is why a board, a roller or any similar object left on a lawn, causes the grass below to turn yellow; it is the reason also why certain parts of plants, not usually green, turn that colour when exposed to the light. Chlorophyll does not always occur in round globules, sometimes it is found in bands. One of the most interesting botanical studies for the microscope is furnished by the leaf of the American water-weed. This plant, which was introduced into the country from North America some years ago, has now spread far and wide and is easily obtained. A leaf which is slightly yellowed with age is the best to take for the experiment. It should be cut from the plant, placed at once in a small bottle of water and kept warm for a few hours; this may be accomplished by keeping the bottle in one’s pocket. After a sufficient interval, put a drop of water in a clean slide, put the portion of leaf in the drop of water, cover with a coverslip and examine with a moderately high power. If the experiment has been properly carried out a wonderful sight will reward us. We shall see that the leaf is divided into a number of divisions called cells; this name has been handed down from the very early days of the microscope, because of a supposed resemblance to the cells in a bee’s honey comb. In each cell we shall see signs of activity, the little round grains of chlorophyll are there, but instead of being stationary, as in the cabbage leaf, they are slowly moving round the walls of each cell. In reality they are carried along in the stream of living matter within the cell. It is a wonderful sight and brings home to the observer very forcibly a fact which is liable to be forgotten, that the plant is just as much a living being as an animal. Perhaps our experiment will not succeed at the first attempt, then we must try again; maybe we have been too rough in detaching the leaf or we have not kept it sufficiently warm. Sometimes the movement may be started by slightly warming the slide over a flame; too much heat, of course, will kill the leaf.