Many and strange are the tales the fossil plants can tell us of the life in the forests when the coal was made, and just as, in the moors, only those moss-plants which were turned to stone will still be there after centuries have gone by, so it was in the old coal-forests that only the plants which were turned to stone remain to tell us their story to-day. For this reason our knowledge of the forests of long ago is not complete; but even now it is enough to tell us something of the life of the plants which were then doing the food-building work of the world. Though the individual plants were so different, the “associations” were in a general way the same as those now living. Great trees reared their heads into the air, and below them, or climbing round and over them, the smaller plants found place long ago as they do to-day. CHAPTER XXXIV. PHYSICAL GEOGRAPHY AND PLANTS If we examine the plants of any district, we find that a number of outside influences affect them very greatly. The most important of these are the physical geography and geology of the place. The form and nature of the rocks and soil, as well as the climate, have a great effect on the plants growing in any spot. You can see this in an extreme case if you imagine yourself up in a balloon looking down on England as on a map. In certain places you see lakes, that is to say, the rocks and soil are so arranged that they form a basin and hold the water permanently there. Now in a lake, as you know, only water-plants will be growing, so that the presence of a fairly deep and constant lake makes it quite certain what kind of plants must grow in that spot. Imagine an earthquake or some slower earth-movement which is strong enough to change the rocks so that the water all runs away, and the result is that there will be dry land in the same spot where before was the lake. This will cause the water-plants to die sooner or later, and land-plants will replace them. There is continual change in the arrangement of lakes and rivers, hills and shores, which takes place all around us, but so slowly that we do not notice it. It is slow, and therefore there is not a sudden killing off of any one kind of plant, and a rapid incoming of a different set of plants, but it causes a gradual shifting and moving of the groups among themselves. Sometimes there may be some swift and sudden change, as the result of a landslip or volcano, or in a stream or lake which has been artificially drained, which shows us a very good object-lesson in plant geography. The importance of the physical form of any place, however, does not only lie in the position of its lakes and streams and the size of its hills. The kind of rock, and nature of the soil covering the rocks, are very important, as well as the many other details of the land.