In this way the shore slowly advances, the floating plants first building up some mud, and the reeds following and bringing shore plants in their train, till in the end the edges of the pond all meet in the middle, and the pond, as such, no longer exists. Only a marsh remains, till this may be gradually grown over by the ever-increasing land-plants, and an oak-tree may grow where once the water-lilies bloomed. If the advancing reeds at the edge had been kept cut back, as they often are, then the land-plants could not have taken such hold, and the pond would have remained a pond with all its “water-weeds.” [Illustration: PLATE VI. LOOKING DOWN ON A SANDY SHORE AND RIVER MOUTH The long spit of sand-dune A protects the marshy land B from the strength of the waves, and here many salt-marsh plants grow. C is the open sea, which at full tide beats on the sandy shore so that no seaweeds or marsh-plants can grow on this side of the dune.] CHAPTER XXXI. ALONG THE SHORE Sandy shores with dunes are so common round Britain that you will probably have opportunities of studying them. Did you ever notice with any care what kind of plants grow on the sand next the sea? As you walk inland from the sea, you will find first little hummocks of sand with a few low, bent grasses, scattered and often far apart. Then as you go a little further inland, the sand mounds are higher, and a stronger grass grows first in tufts and then thickly over them; this grass is the useful sand-binder, or marram grass, and grows on the shifting sand, quite near the sea (~see~ fig. 145). Try to pull up a plant of this grass, and you will probably find out some of the things which help it to hold its position in the moving sand. It is not at all easy to pull up, and you will have to dig rather carefully if you are to get it out at all complete. You will find that what you thought was a simple tuft of grass is really connected, by an underground stem, with other tufts. If you follow this along, you will find that the underground stem runs for a long distance, burrowing in the sand and sending up tufts of leaves at intervals. The tip of the stem always remains under the sand, prepared to grow in whatever direction is best, and unless it is buried to a very great depth it will always continue growing. Coming off from the stem there are very many long roots, and at the places where the leaf tufts arise there are generally one or two much longer and stronger than the others, which run a very great distance into the sand, and if you wish to get them out without breaking them, you may have to dig for several hours. It is by means of these branching underground stems and long roots that the marram grass gets its hold on the sand. When once this grass holds the sand it is soon helped by a number of other plants, which come on behind it and cover the surface, and so prevent the wind from scattering the sand-grains, and blowing them about in clouds.