i04 MARATTIACEAE [CH. 



outermost zone only. The roots, on the other hand, derive their supply from 

 the medullary system : in young plants it springs from the commissural strand, 

 but in the adult mostly from various points on the internal system, though 

 some roots are attached to strands of the outer zone. The roots themselves 

 are often very large, with a massive cortex, and a well-marked stele: this is 

 constructed like that o{ HelinintJiostachys, but in the Marattiaceae the number 

 of protoxylems may rise to ten or even fifteen. 



The structural and developmental details derived from living species are essential for 

 the proper understanding of those fossil stems, often of large dimensions, which have been 

 held as related to the living Marattiaceae. These are grouped under the title Psaronieae, 

 from Psaroniiis, a fossil characteristic of the Lower Permian. Other stems, probably of 

 similar nature but imperfectly preserved as casts or decorticated, have been designated 

 Caidopteris (from the Coal Measures), and Megaphyton and Ptychopteris (from the Upper 

 and Middle Coal Measures). All of these may very probably be Psaronieae. Naturally 

 the specimens will be adult plants, and often they are of very large size. Where their 

 internal structure is preserved the largest of them show extraordinary complexity of the 

 vascular system, which may still be interpreted in terms of the living Marattiaceae, or in 

 general on the same lines as in Ferns at large. But they show only the complex structure 

 of the adult, and this can best find its elucidation through the simpler condition seen in 

 the living Marattiaceae, and ultimately in their sporelings. 



The stems of Psaronius, known as "starling-stones," are found with structure preserved, 

 and often uncrushed. They are characterised by the size of the well-defined central 

 region, which includes the vascular system proper to the stem, surrounded by a thin 

 band of sclerenchyma. In large specimens this may be 4 to 8 inches in diameter. It 

 is surrounded by a zone traversed by innumerable roots, from which, however, outgoing 

 leaf-traces are entirely absent, though their origin is clearly marked in the structure of the 

 central region (Fig. 399). The nature of the peripheral zone has been the subject of much 

 discussion. It consists of roots similar in structure to those of the Marattiaceae, running 

 obliquely downwards, and embedded in a dense filamentous tissue formerly interpreted as 

 cortical. These filaments arose from the outer side of each root, and running radially out- 

 wards came in contact with an outer-lying root. The whole mass of roots was thus welded 

 together into a continuous tissue. In the innermost region, however, similar filaments 

 arose from the surface of the stem itself. Thus the whole zone is made up of adventitious 

 roots embedded in filaments partly of cauline partly of radical origin. The structure is 

 peculiar to Psaroiiius, but it finds an analogue in the hyphal plexus of Fungi and Algae 

 (Scott, Studies, 3rd edn. Vol. i, pp. 273-5). A more general analogy is with the zone of 

 adventitious roots of Cyathea and Dicksonia, which serve a like biological end. But there 

 the roots are not welded together (compare Vol. I, Fig. 152). In either case the roots form 

 an additional support for an upright stem destitute of secondary increase. A late origin 

 of the zone in the ontogeny oi Psaro?ni(s would explain the absence from it of leaf-traces, 

 for it may be assumed that the leaf-stalks would have already fallen away before the outer 

 zone was formed. 



The central region of these fossils is the axis proper. It contains a vascular system the 

 complexity of which is, as in other Ferns, roughly but not exactly in proportion to the size 

 both being in extreme instances very great. The stem of P. brasiliensis (Fig. 399) will 

 serve as a complicated but not extreme example. Embedded in parenchyma limited 

 externally by a sclerotic band, which shows inward extensions and isolated tracts, is the 

 vascular system composed of numerous meristeles disposed regularly ; often these are of 



