156 EDWARD C. JEFFREY ON 



the wi-it('i-'s [lositiou towards these conceptions, as tlie result of tlie stiuhes mentioned 

 above. 



Further since tlie uenus Eqnisetum may he considered in the light of the many and 

 brilliant palaeohotanical reseaix-hes of recent years, as the last degenerate survivor of a 

 phylum, which Ijegins in the petrifactions of the Silurian period, and extends with impor- 

 tant modifications down to the jDresent time, it will be necessary likewise to outline the 

 structure of the various calamitean forms, with special reference to those features, which 

 afford an adequate explanation of the peculiar anatomy of Equiseta, or which on the other 

 hand can themselves only be understood l)y an accurate comprehension of the organization 

 of the living representatives of the calimai-iau stock. 



MoKlMloLOGY OF THE CeXTI.'AI. CYLINDER. 



Sachs (Lehrbuch der botanik, 1882, p. 74 et seq.) in the second edition of his Lehr- 

 buch divided the tissues of the higher plants into three groups, the integumentary, fibro- 

 vascular, and fundamental. De Bary in his Comparative anatomy already referred to 

 adopted the same morphological standpoint. In his conception the fibro vascular strands 

 consist of Nageli's xylem and phloem, and according as the xylem is completely sur- 

 rounded by phloem, or by radially arranged clusters of phloem, or has the phloem applied 

 only to one side, or to two opposite sides, he speaks of concentric, radial, collateral, and 

 bicollateral bundles respectively. 



In recent years Van Tieghem ^Sur la polystelie, Aim. sci. nat. hot., ser. 7, tom. 3, 

 p. 275; Traite de botanique, 1891, p. G73 ; Elements de botanique, 1898, p. 83) has 

 set up a very different hypotliesis of the morphology of the fibrovascular system of the 

 higher cryptogams and the phanerogams. The primitive type of vascular axis, according 

 to his view, consists of a centrally placed stele (concentric fibrovascular bundle of De 

 Bary), limited by a specialized inner layer of the cortex, the endodermis. This simple 

 type of stele, which he calls monostelic, may be modified in various ways. It may for 

 example divide by repeated dichotomy into two, four, eight, etc., exactly similar strands 

 and thus Ijecome polystelie (Van Tieghem, Elements de l)otanique, p. 179 ; Traite de 

 botanique, p. 1370); or secondly, it may become enlarged and develop a central pith and 

 radiating medullary rays, the parenchyma of wdiich, is morjihologically different from the 

 fundamental tissues outside the stele. The fibrovascular strands which appear externally 

 as the result of the formation of the medulla and its rays, are not morphologically com- 

 parable to the concentric steles of the polystelie type. This sort of central cylinder or 

 stele, is found characteristically among the higher vascular plants, but occurs occasionally 

 among the Pteridophyta, e. g. Osmunda and Todea; thirdly, in this type the endodermis 



