Already, in the Early Palæozoic period, the distinction between leaves, stems, roots, and reproductive organs was as clearly marked as it is to-day, and, judging by their structure, they must each have performed the physiological functions they now do. Roots have changed least in the course of time, probably because, in the earth, they live under comparatively uniform conditions in whatever period of the world’s history they are growing. Naturally, between the roots of different species there are slight differences; but the likeness between fern roots from the Palæozoic and from a living fern is absolutely complete. This is illustrated in fig. 35, which shows the microscopic structure of the two roots when cut in transverse direction. The various tissues will be recognized as coming into the table on p. 54, so that both in the details of individual cells and in the general arrangement of the cell groups or tissues the roots of these fossil and living ferns agree. [Illustration: Fig. 35.—A, Root of Living Fern. B, Root of Palæozoic Fossil Fern. _c_, Cortex; _px_, protoxylem in two groups; _m_, metaxylem; _s_, space in fossil due to decay of soft cells.] Among stems there has been at all periods more variety than among the roots of the corresponding plants, and in the following chapter, when the differences between living and fossil plants will be considered, there will be several important structures to notice. Nevertheless, there are very many characters in which the stems from such widely different epochs agree. The plants in the palæozoic forests were of many kinds, and among them were those with weak trailing stems which climbed over and supported themselves on other plants, and also tall, sturdy shafts of woody trees, many of which were covered with a corky bark. Leaves were attached to the stems, either directly, as in the case of some living plants, or by leaf stalks. In external appearance and in general function the stems then were as stems are now. In the details of the individual cells also the likeness is complete; it is in the grouping of the cells, the anatomy of the tissues, that the important differences lie. It has been remarked already that increase in complexity of the plant form usually goes with an increase in complexity of the cells and variety of the tissues. The general ground tissue in nearly all plants is very similar; it is principally in the vascular system that the advance and variety lie. Plant anatomists lay particular stress on the vascular system, which, in comparison with animal anatomy, holds an even more important position than does the skeleton. To understand the essential characters of stems, both living and fossil, and to appreciate their points of likeness or difference, it is necessary to have some knowledge of the general facts of anatomy; hence the main points on which stress is laid will be given now in brief outline.