PTERIDOPHYTA. 713 



E. giganteum (Tropical America) is stated to reach as much as ten metres. The 

 Dutch Rush (E. hiemale) is largely used for polishing owing to the quantities of 

 silica it contains. Characteristic of the stems of Ecjuisetums is the large central 

 air-space, which is only interrupted by diaphragms at the joints. Other spaces also 

 occur associated with the vascular bundles and in the cortex, alternating with the 

 bundles. 



The spore-bearing cones (figs. 403 -• 3- ") consist of stalked, shield-like leaves borne 

 On the club-shaped termination of the axis. The scales bear numerous sporangia on 

 their under surface (tig. 403 *), and in these are the curious and characteristic spores. 

 The wall of the spore is three-layered, and the outmost layer splits away from the 

 one below it, forming four arms attached to the spore at one point (fig. 403'^). 

 These arms, termed elaters (not to be confused with the elaters of Liverworts, cf. 

 p. 696) are extremely hygroscopic, and though at first coiled around the spores 

 (fig. 403*^) become extended as the spores dry, and as their humidity fluctuates 

 contract and expand again. In this way the spores become entangled with one 

 another and are distributed in groups, arm-in-arm. The importance of this circum- 

 stance appears to be as follows : — The spores, though all of one sort {i.e. homosporous), 

 give rise to dioecious prothallia as a rule (cf. fig. 403^, representing a male pro- 

 thallium); consequently it is of advantage for promoting fertilization that a number 

 of prothallia should arise in the same neighbourhood. This result is achieved by a 

 linking of the spores. The prothallia are richly lobed, but not unlike those of Ferns. 



The Calamarice are found as common fossil remains in the carboniferous forma- 

 tions. They include casts of the medullary cavity, impressions of the surface, and 

 actual portions of the stems and cones in a petrified state. Many members of this 

 family attained gigantic proportions, and their stems underwent a well-marked 

 secondary increase in thickness. An examination of the cones shows that these 

 former Equisetales possessed both micro- and macrospores. 



Alliance XXV. — Lycopodiales, Club-mosses. 



Forms usually with elongated, branching stems and small leaves distributed over 

 them. The sporangia are borne on the upper surface of the leaf or in the leaf-axil; 

 the fertile leaves are in many cases aggregated into cones. Both homosporous and 

 heterosporous families occur. 



Families: Lycopodiacece, Psilotacew, Selaginellacece, Lepidodendracece, Sigilla- 

 riacecc, Isoetacece. 



Whilst in the Filices and Equisetales several or many sporangia are present on 

 the fertile leaves, in this alliance there is only one, and this is situated on the upper 

 surface or in the leaf-axil. The sporangia in this group difier from those in many 

 of the Filices {e.g. Polypodiaceae) in being more massive and in liaving origin not 

 from single epidermal cells, but from a row or group. Their form also is in many 

 cases peculiar. The LycopodiacejB and Psilotacese are homosporous, the other families 

 heterosporous. In the former the prothallia generally resemble those of Ferns, in 



