Look at the “shade leaves” of any ordinary tree, such as a Lime or Maple, and compare them with those growing in the sunlight, even on the same tree. They are larger and softer and thinner. To absorb the same amount of energy as the more brilliantly lighted leaves, they must expose a larger surface to the light. Hence if the Coal Measure plants grew in very great shade, to supply their large growth with the necessary sun energy we should expect to find enormous spreading leaves. But what is the fact? No such large leaves are known. _Calamites_ and _Lepidodendron_, the commonest and most successful plants of the period, had narrow simple leaves with but a small area of surface. They were, in fact, leaves of the type we now find growing in exposed places. The ferns had large divided leaves, but they were finely lobed and did not expose a large continuous area as a true “shade leaf” does; while the height of their stems indicates that they were growing in partial shade—at least, the shade cast by the small-leaved Calamites and Lepidodendrons which overtopped them. Indeed there is no indication from geological evidence that so late as Palæozoic times there was any great abnormality of atmosphere, and from the internal evidence of the plants then growing there is everything to indicate a dry or physiologically dry[14] sunny condition. Of the plant fossils from the Coal Measures we have at least two types. One, those commonly found in nodules _in_ the coal itself; and the other, nodules in the rocks above the coal which had drifted from high lands into the sea. The former are the plants which actually formed the coal itself, and from their internal organization we see that these plants were growing with partly submerged roots in brackish swamps. Their roots are those of water plants (see p. 150, young root of Calamite), but their leaves are those of the “protected” type with narrow surface and various devices for preventing a loss of water by rapid transpiration. If the water they grew in had been fresh they would not have had such leaves, for there would have been no need for them to economize their water, but, as we see in bogs and brackish or salt water to-day (which is physiologically usable in only small quantities by the plant), plants even partly submerged protect their exposed leaves from transpiring largely.