IX] ANATOMY OF EQUISETUM. 253 



and this is followed by a distinct endodermis, of which the cells 

 show the characteristic black dot in the cuticularised radial 

 walls (fig. 52, D). Beyond the endodermis there is the large- 

 celled parenchyma of the rest of the cortex. Tannin cells occur 

 here and there scattered among the ground tissue. On the 

 same radius on which each vascular strand occurs, the cortical 

 parenchyma passes into a mass of sub-epidermal thick-walled 

 mechanical tissue or stereome. Alternating with the ridges of 

 stereome, the grooves are occupied by thin-walled chlorophyll- 

 containing tissue w^hich carries on most of the assimilating 

 functions, and communicates with the external atmosphere by 

 means of stomata arranged in vertical rows down each internode. 

 The continuity of the cortical tissue is interrupted by the 

 occurrence of large longitudinal vallecidar canals alternating in 

 position with the stem ridges and vascular strands (fig. 52, C, v). 

 The epidermis consists of a single layer of cells, containing 

 stomata, and with the outer cell-walls impregnated with silica. 



In certain species of Equisetum, e.g. E. palustre L., the whole 

 circle of vascular strands is enclosed by an endodermis, and has 

 the structure typical of a monostelic stem. In others e.g. E. 

 litorale Kiihl. each vascular strand is surrounded by a separate 

 endodermis, and in some forms e.g. E. silvaticum L. there is an 

 inner as well as an outer endodermal layers Without discussing 

 the explanation given to this variation in the occurrence of the 

 endodermis, it may be stated that in all species of Equisetiun 

 the stem may be regarded as monostelic^. 



In the rhizome the structure agrees in the main with that 

 of the green shoots, but the vallecular canals attain a larger 

 size, and the pith is solid. A slightly enlarged transverse sec- 

 tion of a rhizome of Equisetum maximum is shown in fig. 55, 2, 

 the small circles surrounding the pith mark the position of 

 the vascular bundles and carinal canals; the much larger spaces 

 between the central cylinder and the surface of the stem are the 

 vallecular canals. 



The central cylinder or stele of the root is of the diarch, 

 triarch or tetrach type ; i.e. there may be 2, 3 or 4 group.s of 



1 Pfitzer (67). * Straeburger (91) p. 448. 



