598 GEOLOGY. 



enough, in undrained swamp plants. The trunks arose from stout 

 underground rhizomes. The root structure was of the type commonly 

 found under water or in wet mud. The reproductive organs were in 

 spikes or strobiles, and gave evidence of higher development than now. 

 The calamites were probably associated in thickets or jungles of the 

 cane-brake or bamboo type. Grand' Eury reports great forests of them 

 in situ in central France. They probably frequented swamps and 

 lowlands. As the calamites were well differentiated in the Devonian 

 their history may run far back. Their derivation is uncertain, but 

 the next group throws much light on their relations. 



The Sphenophyllales. — Recent studies have shown that the grace- 

 ful, slender plants with whorled leaves, referred to the genus Spheno- 

 phyllum (Fig. 279, c), and formerly classed as calamites, differ in impor- 

 tant features from all calamariajis, and from the other great divisions 

 of the pteridophytes, and form a class by themselves. Their import- 

 ance lies chiefly in the fact that while they have certain calamarian 

 features, they have others possessed by the lycopods, which is inter- 

 preted to mean that these two groups were united with the Spheno- 

 phyllales in a common ancestral form. 1 The stems were long, slender, 

 and apparently weak, and so a climbing habit has been inferred. The 

 leaves were without palisades, suggestive of a shady habitat, perhaps 

 one of undergrowth. The class was represented in the Devonian, 

 had its climax in the middle Coal Measures, and continued into the 

 Permian and possibly later. 



The Lycopodiales. — This was the master group of the Coal flora, 

 constituting trees of large size and attaining to the highest organi- 

 zation reached by the pteridophytes. From that high estate, they 

 have since fallen to prostrate or weakly ascending plants of moss- 

 like aspect (club mosses, ground pines). The chief genera were Lepi- 

 dodendron and Sigillaria, of which the former was the earlier and sim- 

 pler type. Both take their names from the leaf -scars or leaf -cushions 

 (lepidos = scale, sigilla ■= seal) which the trunks retained with much 

 persistency, implying little or no exfoliation, though the trunks were 

 affected by secondary growth. The scars are for the most part the 

 leaf-bases or leaf-cushions, rather than the actual scar, which occupies 

 but a small part of the cushion. In the lepidodendrons they are ar- 

 ranged spirally (Fig. 280); in the sigillarians, vertically (Fig. 281). 



1 Seward, Fossil Plants, p. 413. Scott, Studies in Fossil Botany, p. 494. 



