KHIZOME OF PTERIS. 173 



A cross-section through a stout creeping rhizome of the 

 Bracken, examined with the naked eye or with a lens, shows it more 

 or less elliptic in section, and covered externally by a thick rind 

 of a dark brownish-yellow colour, which microscopic study would 

 show to be an epidermis and a pseudo-sclerenchymatous layer, the 

 aggregate colour in which is due to the natural bright yellow 

 coloration of the cell- walls. Within this rind is the general mass 

 of the ground- tissue, an opaque white in colour, and very sappy 

 in fresh material. Two dark brownish-yellow bands can be seen 

 in this, of unequal size, one nearly flat and the other arched, and 

 dividing the general ground-tissue into an inner and outer portion, 

 communicating between the ends of the bands ; these are bands 

 of internal sclerenchyma, and in some cases, especially near the 

 point of branching of the rhizome, these bands join into one, of 

 horse- shoe form. Both within and without these internal scler- 

 enchyma bands, and embedded in the ground-tissue, will be seen 

 what subsequent study will show to be the vascular strings, 

 golden-yellow in colour, and each enclosed in a delicate brownish 

 line. These vascular strings were formerly known as vascular 

 bundles ; they are steles, and, as we shall see, each is enclosed 

 by a starch sheath, and an outer layer of cells of the ground- 

 tissue constituting an endodermis. The steles, however, from 

 time to time coalesce one to one, so that, if their longitudinal 

 course were traced, they would be seen to form a very irregular 

 network ; hence while the rhizome is considered to be polystelic, 

 the steles may be considered as schizosteles when they separate, 

 or gamos teles when they coalesce. The stem of a very young 

 fern only shows one central stele, and in this respect strikingly 

 resembles the root of the same plant. If then each of these 

 vascular strings is a stele, the general ground-tissue in which 

 they are embedded is manifestly equivalent to that which we 

 have called " cortex " in phanerogamic stems and roots. The 

 term ''cortex" is, indeed, . sometimes applied to it in the Fern; 

 but inasmuch as this term has acquired a specific meaning, as an 

 external enveloping layer, or rind, the application of the same term 

 to this case appears undesirable. I [ED.] have already said that 

 in many Dicotyledonous stems the limits of the central cylinder 

 are not recognisable ; it does not follow that the limits are not 

 present, but if we suppose for the moment that each vascular 

 bundle of such a stem were in itself a stele, the cortex, medullary 

 rays and pith would form a collective ground-tissue of a kind 



