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_ Kühl. each vascular strand is surrounded by a separate endodermis, and in some forms _e.g._ _E. sylvaticum_ L. there is an inner as well as an outer endodermal layer[492]. Without discussing the explanation given to this variation in the occurrence of the endodermis, it may be stated that in all species of _Equisetum_ the stem may be regarded as monostelic[493]. 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 section 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 tetrarch type; _i.e._ there may be 2, 3 or 4 groups of protoxylem in the xylem of the root stele. The axial portion is occupied by large tracheids, and the smaller tracheids of the xylem occur as radially disposed groups, alternating with groups of phloem. External to the xylem and phloem strands there occur two layers of cells, usually spoken of as a double endodermis, but it has been suggested that it is preferable to describe the double layer as the _phloeoterma_[494], of which the inner layer has the functions of a pericycle, and the outer that of an endodermis. A transverse section of a root is seen in fig. 55, 1, the dark cells on the left are part of a thick band of sclerenchyma in the cortex of the root, the layer e is the outer layer of the phloeoterma. Without describing in detail the development[495] of the sporangia, it should be noted that the sporangial wall is at first 3 to 4 cells thick, but it eventually consists of a single layer. The cells have spiral thickening bands on the ventral surface, and rings on the cells where the longitudinal splitting takes place. Each sporangium is supplied by a vascular bundle which is given off from that of the sporangiophore axis. The strobili are isosporous. ╭ A. EQUISETITES. │ B. PHYLLOTHECA. II. FOSSIL EQUISETALES. ┤ C. SCHIZONEURA. │ D. CALAMITES. ╰ E. ARCHAEOCALAMITES.