If you live anywhere in the south of England, walk over some part of the downs till you see below you in the valley a clay-pit or pottery factory, which shows you that the chalk is no longer under the surface soil, but that it has been replaced by clay. Walk straight towards this place, collecting the plants you meet on the way. On the actual downs you will find many which do not grow near the clay-pit, since they are special chalk lovers. In the clayey valley it is very likely that you may find a pond; if so, walk towards it, noting all you pass on the way. As you get to the edge, reeds and bulrushes, water forget-me-not, tall spikes of water loosestrife, and many others appear which you would have been astonished to meet with on the downs. [Illustration: Fig. 154. A recently formed pond in Delamere with a dead forest tree standing up in the middle.] A very important factor also is the amount of rain which the district gets. This tells particularly among the ferns and mosses. Along the hedgerows of Kent, for example, where it is rather dry, true ferns will seldom grow, while in Devonshire every hedge and bank has many hundreds of the common polypody fern and the hartstongue. When we come, however, to consider on what it is that the rainfall depends we find that it is the structure, size, and relations of the land masses to the sea and the winds. In fact, it depends on the physical geography of England as a whole. So that in the end the plants and the physical nature of any place are so much in touch that it is almost impossible to do anything in the study of plant distribution without considering physical geography. [Illustration: Fig. 155. A recently formed pond which has covered a large area of the forest and killed many of the trees. Notice the dead trunks standing and lying about, and the rushes growing near the edge, which would not have been there but for the coming of the water.]