XXIIl] PSARONIUS 417 



V-shaped meristeles or closed oval cylinders. As a leaf- trace passes out 

 compensating strands occupy the foliar gap. 



The vascular region is surrounded by a parenchymatous cortex, which 

 in younger plants, or in the apical region of an older plant, forms the 

 surface of the stem to which the leaf-stalks are attached. From the 

 peripheral steles, or from the more external band.s of the vascular network, 

 roots are given oflF which pass in a sinuous vertical course through the 

 cortex, appearing on the surface between the leaf-bases. In older stems, 

 after leaf-fall, the tissue immediately external to the vascular region 

 produces secondary parenchyma with which the roots become intimately 

 associated by their outermost cells. As a result of the secondary cortical 

 development and the gradual increase in the number of roots invading 

 the cortical tissue from above, the stem is enclosed by a cylinder of roots 

 and associated parenchymatous tissue of secondary origin. In still older 

 portions of a stem the more external roots are free from the stem-cortex 

 and form a thick felted mantle, which increases in thickness towards 

 the base of the tree. 



The roots (fig. 296, E) are polyarch, 5 — 10 groups of xylem alternating 

 with strands of phloem, and similar in structure to those of recent species 

 of Marattia and Angiopteris; the stele is enclosed by an inner cortex of 

 compact or lacunar tissue containing secretory sacs, and this is surrounded 

 by a cylmder of mechanical tissue. In one or two instances secondary 

 xylem has been observed wholly or partially enclosing the root-stele i. 



Our knowledge of the anatomy of Psaronius is based largely 

 on the investigations of Stenzel considerably extended by 

 Zeiller's more intensive studies and, more recently, by the later 

 work of Stenzel^ and that of Rudolph. A striking fact, which 

 has led to various suggestions, is that in a transverse section of 

 a Psaronius stem with its encasing cylinder of roots no signs of 

 leaf-traces are met with in the root-region. If the roots simply 

 penetrated the cortex, as in some recent species of Lycopodium 

 (fig. 125, A) or as in Angiopteris, we should expect to find 

 leaf-traces in the outer region (root-cylinder) of Psaronius 

 stems. An explanation of the absence of leaf-traces which was 

 suggested by Stenzel, is that the cortical zone formed a 

 comparatively narrow band in the young leaf-covered stem; 

 after leaf-fall it became the seat of active growth in its inner 

 layers and so produced a constantly widening zone of secondary 

 parenchyma, which pushed the superficial cortical tissue with 



1 Butterworth (00). Pelourde (08^) has recently described the structure of 

 the roots of several species of Psaronius. 



2 Stenzel (06). 



