386 EQUISETALES 



large pith with a central cavity interrupted by diaphragms at the 

 nodes : around it is disposed a ring of vascular strands of number varying 

 according to the species, or according to the rank of the axis in question. 

 They are separated laterally by broad parenchymatous rays, while the 

 whole is surrounded in most species by a continuous endodermis 

 (Fig. 2ii A, B). There is, however, a good deal of difference in the 

 disposition of the endodermis in various species, and these differences are 

 of such a nature as to raise questions as to the validity of the simple 

 character of the stele itself. The simplest case is that above described, 

 and it may be seen in the aerial shoots of E. arvense and palustre, where 

 there is a simple endodermal sheath of sinuous outline, formed from the 

 innermost layer of the cortex ; in fact, the arrangement is that most usual 

 in Vascular Plants. In this case the term " stele " will naturally connote 

 all that lies within that sheath. A second type is that seen in the 

 rhizomes, but not in the aerial shoots of E. sylvaticum (Fig. 211 c, D), 

 in which a second endodermis is present as a sinuous layer, forming an 

 inner barrier of demarcation from the inner-lying pith. A third type is 

 seen in the rhizomes of E. hiemale and some others, but not in the aerial 

 stems of those species : it is characterised by each single strand being 

 individually surrounded by a closed endodermal sheath (Fig. 211 E, F), 

 while there is no general endodermis delimiting the whole stele. Such 

 individual endodermal sheaths also surround the strands in the tubers of 

 E. arvense, sylvaticum, and palustre, species in which, however, a general 

 endodermis is found in the ordinary axes. The inconstancy of the 

 arrangements thus seen, even in the different regions of the same plant, 

 indicates them as special and secondary peculiarities, which need not 

 seriously affect the conception of the stem as essentially monostelic. The 

 fact that the differences of the endodermis do not otherwise affect the 

 anatomy confirms this conclusion. It may then be held that the stem 

 of Equisetum is monostelic throughout, but subject to disintegration of 

 the stele. 



The structure of the individual vascular strands, as seen in the transverse 

 section of the internode, is fairly uniform in the different species. Each 

 strand shows towards its central limit a canal designated "carinal," 

 because it is on the same radius as one of the keel-flanges which mark 

 the fluted internode externally (Fig. 211 c). These canals indicate the 

 position of the protoxylem-strands, which become obliterated as the sur- 

 rounding tissues expand in development; for the primary tracheides are unable 

 to keep pace in their own growth with the expansion of the surrounding 

 tissues, and accordingly break down. Close on either side of the margin 

 of each carinal canal the annular thickenings of one or two or more 

 tracheides remain to maturity, and permanently record the position of the 

 protoxylem. As we shall see later, these are directly continuous with 

 the protoxylem of the leaf-trace. Further out from the centre than the 

 canal, and right and left of it, two other groups of xylem arise later : 



