XX] ANATOMY 105 



large and varying form. The leaf-arrangement in this plant was decussate, and leaf-traces 

 are^seen departing at Fi, F2, though not exactly at the same level. F2 shows the out- 

 going leaf-trace as a cor\tinuous curve open inwards. The long tracts A, B lying laterally 

 are tlie leaf-traces of the next pair of leaves, A E of the next higher pair. These have 

 been called the "reparatory" meristeles to distinguish them from the four long and curved 

 "peripheral" meristeles, P 1-P4, that lie diagonally. These give rise to the root-supply, 

 and after anastomosis with the reparatory tracts they form the leaf-traces. At C a tract 



Fig- 399- Psaronius brasiliensis. Transverse section of stem, 

 whole of the true stem, containing the vascular bundles, is shown, 

 together with a great part of the outer zone with innumerable adven- 

 titious roots. F\, F2, leaf-gaps. See Text. (Reduced, after Zeiller, 

 from Scott.) 



is seen separating from the peripheral meristele to pass to the reparatory tract, as a con- 

 tribution to the leaf-trace. The meristeles lying internally form a cauline system, as in 

 other large Fern-stems. It is thus seen that a large Psaronius corresponds in essentials 

 to a very complicated polycyclic Fern. The type of structure is suggested, though on a 

 very much smaller scale, by Saccoloma (Vol. I, Fig. 146, p. 153). 



Stems which are evidently Psaronioid show a great variety in size, structure, and leaf- 

 arrangement. On the latter point Zeiller distinguished three types: the Polystichi, with 

 numerous vertical rows of leaves (Fig. 400, A): the Tetrasiichi, with four series (Fig. 

 399): and the Distichi with only two (Fig. 400, D). Upon this feature the detail of the 

 vascular system naturally depends. But it is also closely related to size. The actual 

 dimensions are not always stated by authors, but in the following examples (p. 107) they 

 are believed to be approximately correct. 



In discussing such questions in fossils, the interest at once centralises on P. Renaulii, 

 from the Lower Coal Measures (Fig. 401). Its dimensions are approximately those of any 

 ordinary solenostelic rhizome among living Ferns. There can be no doubt of its Psaronial 

 nature (compare Williamson, Phil. Trans. 1876, PI. III). It contains a solenoxylic stele, 



