Another thing that should go with the mapping is the plotting out of “sections” through the irregular land, which will show clearly how frequently the plants growing on any spot are determined by the level of the spot and its consequent relation to the water supply. The most striking case of this kind is that of a section through a pond or stream and its banks. Unless you have a boat at your service, you will have to choose a stream where two people can meet across it from the banks, or else content yourself with going out only as far as you can wade. To begin the “section” you should choose a good place where there seems to be plenty of variety in the plants; then fix a strong stake into the water as far out as you intend to go, tying on to it a string measured out into 1-foot divisions. This string should be 20, 30, or more feet long, according to the kind of edge the pond has, and its other end should be fastened to a stake also. [Illustration: Fig. 156. A “section” of the edge of a pond plotted out on mathematical paper. ~a-b~, the level of the water. A-B, a line parallel to it, marked by a measured string fastened to stakes, from which the measurements are taken.] Take the dry land where ordinary land-plants are growing as your starting-point, and fasten your string to it, as in (B) fig. 156, making it level on your stake in the water (A) if possible, so that the same string can be used to take measurements and levels from. As you work measure the actual distance ~along~ the string, and the ~depth from~ the string of each variety of plants, and where there are few, of each individual plant crossed by the string. When you come to plotting this out on mathematical paper you will require to reduce the scale by letting two small squares of the paper represent an actual foot, or whatever seems to be convenient. Then from your actual measurements you can soon plot out a “section” of the pond, e.g., in actual measurements the bulrushes were growing 1 foot below the water surface, that is, 4-1/2 feet below the fixed level, and the first was 6-2/3 feet distant from the stake in the water. In plotting you should represent the actual plants by symbols or simple signs, as is done in the figure, so as to be able to see at a single glance just how everything was arranged. Note also the level of the surface of the water, which you may choose as your working level if you prefer it to the line given by A-B. From this you will see very clearly how extremely important is the amount of water in determining what kind of plant is growing in any given spot. After having done these small pieces of mapping, other problems will suggest themselves to you, and you will find that the work of making maps and plans of the plants is more than repaid by the facts you find out from the plants themselves, and the insight you get into some of the rules which guide the plants in their choice of their homes. CHAPTER XXXVI. EXCURSIONS AND COLLECTING