The similarity between the living cells and those of the Coal Measure fossils is sufficiently illustrated to need no further comment. This similarity is an extremely helpful point when we come to an interpretation of the fossils. In living plants we can study the physiology of the various kinds of cells, and can deduce from experiment exactly the part they play in the economy of the whole plant. From a study of the tissues in any plant structure we know what function it performed, and can very often estimate the nature of the surrounding conditions under which the plant was growing. To take a single example, the palisade tissue, illustrated in fig. 25, _p_, in living plants always contains green colouring matter, and lies just below the epidermis, usually of leaves, but sometimes also of green stems. These cells do most of the starch manufacture for the plant, and are found best developed when exposed to a good light. In very shady places the leaves seldom have this type of cell. Now, when cells just like these are found in fossils (as is illustrated in fig. 34), we can assume all the physiological facts mentioned above, and rest assured that that leaf was growing under normal conditions of light and was actively engaged in starch-building when it was alive. From the physiological standpoint the fossil leaf is entirely the same as a normal living one. [Illustration: Fig. 34.—From a Photo of a Fossil Lea _e_, Epidermis; _p_, palisade cells; _pr_, soft parenchyma cells (poorly preserved); _s_, sclerenchyma above the vascular bundle.] From the morphological standpoint, also, the features of the plant body from the Coal Measure period fall into the same divisions as those of the present. Roots, stems, leaves, and reproductive organs, the essentially distinct parts of a plant, are to be found in a form entirely recognizable, or sufficiently like that now in vogue to be interpreted without great difficulty. In the detailed structure of the reproductive organs more changes have taken place than in any others, both in internal organization and external appearance.