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.
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.
Although the changes in physical geography which made and unmake continents are slowly acting around us all the time, it is not often that we can clearly see them taking effect. Photos 154 and 155 are therefore particularly interesting, for they show one of the processes at work. Part of a forest is in the actual course of being killed by the pond which is forming on sinking land. This pond and several smaller ones of the same kind can be studied in the neighbourhood of Delamere forest, in Cheshire. Here the under soil gets washed out in certain places, and the surface earth sinks and forms a hollow in which water collects. In fig. 154 you see one tree standing in the middle of the pond. It is dead, and has been killed by the water (you remember that ordinary plants are drowned by too much water) and in fig. 155 you see a large area entirely covered with water, and the dead trees standing up through it. This pond is spreading rapidly, and is a good illustration of the reverse condition from that seen in fig. 144, where the plants by their growth are filling up a pond. The washing out of the soil and the collecting of the water in this case was quite beyond the control of the plants themselves, but they are supremely affected by it.
CHAPTER XXXV.
PLANT-MAPS
In the last chapter we noticed a few of the many facts which show us that a close relation exists between the plants and the nature of the land on which they grow. We may now try to express these facts in a simple way by making maps of the land according to the plants growing on it.