III. EQUISETACE^E. 



905 



Equisetum. 

 Antherozoids. (Thuret.) 



Equisetum flueiatile. 

 Fructiferous spike. 



Equisetum. 



Arcliegonium (mag.) 



(Duval-Jouve.) 



Equisetum limosum. 

 Prothallus developed (mag.). (Thuret.) 



Perennial terrestrial or aquatic PLANTS ; rhizome subterranean, running 1 , often 

 branched, covered with brown hairs, joints sometimes bulbous. STEMS straight in 

 evolution, jointed, formed of cylindric regularly furrowed internodes, and terminated 

 above in a ring, of which the free edge is elongated as a toothed sheath. A similar 

 sheath terminates also the joints of the rhizome. Internodes of the stem fistular, 

 closed above by a cellular diaphragm opposite the base of the sheath. Walls of the 

 internodes composed of 2 concentric cylinders ; outer or cortical cylinder entirely 

 fibro-cellular, and usually presenting large longitudinal cavities opposite the furrows 

 of the internode; inner cylinder formed of bundles of annular or spiral vessels, and 

 presenting small longitudinal cavities opposite the ridges of the internode, and con- 

 sequently alternate with the cavities in the cortical cylinder. The number and 

 arrangement of these cavities (best seen in a transverse section of the stem) are a 

 certain guide to the determination of the species, which are divided into vernal and 

 cestival, according to the period at which fructification takes place. The stems are 

 simple, or furnished with regularly whorled branches, invariably placed below the 

 nodes and the commencement of the sheaths; these branches, and the whorled 

 branchlets which they sometimes bear, resemble the stem in structure, but in some 

 species they want the central cavity and the cortical spaces ; the cavities and fibro- 

 vascular bundles of the inner cylinder, are however, always present ; the same is the 

 case with the internodes of the rhizome. The epidermis of the stem, branches, and 



