PTERYDOPHYTA 939 



with about 25 species, and the extinct Calamites, represent a range 

 in height from a few inches in the modern forms to from 30 to 60 

 meters in the extinct Calamites. Equisetum arises from a subter- 

 ranean rhizome, which may be a meter in length, and is jointed : 

 the aerial shoot consists of hollow internodes, with whorls of 

 leaves near the top of each, the leaves cohering, except near their 

 tips. In section the aerial stem shows a hollow central cylinder. 

 around which is arranged a circle of fibrovascular bundles, triangu- 

 lar in section, with the point inward. The inner end is occupied by 

 a large air space, and outside of this again is a circle of long air 

 tubes alternating with the fibrovascular bundles. These latter 

 extend into the leaves, equaling in number the leaf teeth. 



The stem of the extinct Calamites had essentially the same 

 structure, but with secondary growth in thickness. In all large 

 specimens a broad zone of wood is added, with a structure compar- 

 able in the true Calamites to that of the simplest conifers. The 

 vascular bundles project into the pith as in Equisetum, and from 

 their more resistant character they will remain when the pith 

 breaks down. A rock-filling of the hollow cylinder thus made will 

 be marked by longitudinal grooves, representing the projecting 

 vascular bundles. In Calamites proper these grooves alternate at 

 the nodes, while in ArcliKOcalamites they are continuous. This 

 shows that in the latter the leaves were superposed, v/hile in Cala- 

 mites they were alternating. In modern Equisetum both fertile 

 and sterile branches arise from the rhizomes. The sterile are more 

 slender than the spore-bearing ones, and bear numerous whorls of 

 branches, which form a bushy plant, from which the name "horse- 

 tail" originated. The fertile branches bear a terminal "strobilus," 

 or cone of sporangiophores, each of which consists of a hexagonal 

 disk, attached by a stem to the axis and supporting on its under 

 side six to nine large spore-cases or sporangia. The outer surfaces 

 of the hexagonal plates form the solid outer surface of the cone, 

 the sporangia extending inward toward the axis. They are not 

 visible until the cone separates into its component parts. Some 

 Calamites ( Archaeocalamites) agree closely with this mode of 

 organization, but in others the structure of the cones was more 

 complicated, this being brought about chiefly by the insertion of 

 whorls of sterile bracts between those of the sporangiophores. 



The Sphcnophyllincc, known only from the Palaeozoic, and rep- 

 resented by the genus Sphenophyllum, had some characters of the 

 Equisetales. The slender, little-branched, and probably clinging 

 stem had from six to eighteen wedge-shaped or linear leaves at 

 the swollen nodes, the leaves of successive whorls not alternating. 



