The Pteridophytes include plants which vary in size from a few millimetres[482] to several metres in height. The spore on germination gives rise to a small thalloid structure, the _prothallium_, on which the sexual organs are developed; this is the _gametophyte_ or sexual generation. The sexual organs have the form of typical archegonia and antheridia. From the fertilised egg-cell there is developed the Pteridophyte plant or _sporophyte_, which bears the spores. This asexual generation shows a well-marked external differentiation into stem and leaves, and bears true roots. Internally the tissues exhibit a high degree of differentiation into distinct tissue-systems. True vascular bundles occur, which may or may not be capable of secondary thickening by means of a _cambium_, _i.e._ a definitely localised zone of meristematic tissue. The sporangia are borne either on the ordinary foliage leaves or on special spore-bearing leaves called _sporophylls_, which differ in a greater or less degree from the sterile leaves. The majority of the best known and most important Palaeozoic genera are either true Vascular Cryptogams, or possess certain of the pteridophytic characteristics combined with those of higher plants. It is not merely the commoner and more familiar recent genera with which the student of extinct types must be acquainted, but it is extremely important that he should make himself familiar with the rarer, less known and more isolated recent forms, which often throw most light on the affinities of the older representatives of the group. It is often the case, the more isolated living plants are, the more likely are they to afford valuable assistance in the interpretation of genera representing a class, which reached its maximum development in the earlier periods of the earth’s history. The importance of paying special attention to such recent plants as may be looked upon as survivals of a class now tending towards extinction, will be more thoroughly realised after the extinct vascular cryptogams have been dealt with.