The individual plants of the Coal Measure period differed entirely from those now living; they were more than merely distinct species, for in the main even the families were largely different from the present ones. Nevertheless, when we come to examine the minute anatomy of the fossils, and the cells of which they are composed, we find that between the living and the fossil cell types the closest similarity exists. From the earliest times of which we have any knowledge the elements of the plant body have been the same, though the types of structures which they built have varied in plan. Individual _cells_ of nearly every type from the Coal Measure period can be identically matched with those of to-day. In the way the walls thickened, in the shapes of the wood, strengthening or epidermal cells, in the form of the various tissues adapted to specific purposes, there is a unity of organization which it is reasonable to suppose depends on the fundamental qualities inherent in plant life. This will be illustrated best, perhaps, by tabulating the chief modifications of cells which are found in plant tissues. The illustrations of these types in the following table are taken from living plants, because from them figures of more diagrammatic clearness can be made, and the salient characters of the cells more easily recognized. Comparison of these typical cells with those illustrated from the fossil plants reveals their identity in essential structure, and most of them will be found in the photos of fossils in these pages, though they are better recognized in the actual fossils themselves. Principal Types of Plant Cells Epidermal. [Illustration: Fig. 21 _Epidermis._—Protecting layer or skin. Cells with outer wall thickened in many cases (fig. 21, _a_ and _b_). Compare fossil epidermis in fig. 34, _e_.] [Illustration: Fig. 22 _Hairs._—Extensions of epidermis cells. Single cells, or complex, as fig. 22, _h_, where _e_ is epidermis and _p_ parenchyma. Compare fossil hairs in figs. 79 and 120.] [Illustration: Fig. 23 _Stomates._—Breathing pores in the epidermis. Seen in surface view as two-lipped structures (fig. 23). _s_, Stomates; _e_, epidermis cells. Compare fossil stomates in fig. 8.] Ground Tissue [Illustration: Fig. 24 _Parenchyma._—Simple soft cells, either closely packed, as in fig. 24, or with air spaces between them. Compare 78, B, for fossil.] [Illustration: Fig. 25 _Palisade._—Elongated, closely packed cells, _p_, chiefly in leaves, lying below the epidermis, _e_, fig. 25. Compare fig. 34, _p_, for fossil palisade.] [Illustration: Fig. 26 _Endodermis._—Cells with specially thickened walls, _en_, lying as sheath between the parenchyma, _c_, of ground tissue, and the vascular tissue, _s_, fig. 26. Compare fig. 108 for fossil endodermis.] [Illustration: Fig. 27