XVIl] STIGMARIA 245 



phelloderm of Lepidodendron. The surface of this older rhizome 

 retains patches of primary tissue, and an occasional rootlet, as at 

 r, fig. 210, E, is seen in connexion with the cortex ; the cortex 

 has been vertically fissured as the result of secondary growth 

 and presents an appearance like that shown in Lepidodendron 

 Wwnschianum and L. Veltheimianum (figs. 181, A, and 186, A). 

 The form in which a Stigmarian rootlet is usually preserved 

 is shown in fig. 210, D ; the single vascular bundle strand with 

 its endarch protoxylem (fig. 210, B, px) is enclosed by a ring of 

 inner cortical parenchyma (fig. 210, F c^) ; the cells in immediate 

 contact with the xylem having usually disappeared. Beyond 

 the middle cortical space a second cylinder of parenchyma 

 represents the outer cortex (F, &) in which a layer of dark- 

 walled cells (6, fig. 210, F) may be compared with the hypo- 

 dermal band in the main Stigmarian axis (G, b). These 

 Stigmarian rootlets, usually less than 1 cm. in diameter, are the 

 commonest objects in sections of the calcareous nodules from 

 English coal-seams. A good example of their abundance is 

 shown in fig. 210, D and E ; here they have invaded the space 

 formed by the splitting of the secondary cortical tissues along 

 the line of the phellogen and a few are seen here and there in the 

 deeper layers of the phelloderm (s, fig. 210, E). Not infrequently 

 the close contact of these ubiquitous rootlets with the tissues of 

 the plant which they have invaded leads to confusion between 

 invader and invaded. Partially decayed tissues lying, probably, 

 under water were penetrated by Stigmarian rootlets in exactly 

 the same way as the roots of recent plants bore through vege- 

 table substances which happen to be in their path. The rootlet 

 bundles are in the first instance composed of the primary tracheae 

 which line the inner edge of the secondary xylem ; these receive 

 additions from the meristematic zone, and thus, when seen in 

 the cortex outside the stelar region, are found to consist in part 

 of primary and in part of a fan-shaped group of secondary 

 tracheae. On the other hand, the monarch bundle as it 

 appears in a free rootlet is usually composed entirely of 

 primary elements (fig. 210, A— C, F). It has been shown by 

 Weiss 1 that in the Stigmarian rhizome of what is probably 



' Weiss, F. E. (02). 



