372 



EOUISETALES 



separated by leaf-sheaths (Fig. 196). It is thus seen that the strobilus of 

 Equisetum is not always that circumscribed terminal body which is typical 

 for the living species. 



The Equisetum-^^ has been recognised, though with some uncertainty, 

 and only in few specimens, as far ^back as the Middle Coal Measures ; l 



but it is seen represented more commonly, 

 and by large forms, in the Mesozoic rocks. 

 Related to it are two other fossil forms : 

 the genus Phyllotheca of Permian age 

 resembles Equisetum in the general features 

 of the shoot, with its cup-like leaf-sheaths 

 webbed at the base, but differing in the 

 form of the leaves and in the fertile 

 region : this is constructed on the general 

 plan of Equisetum, but with the strobilus 

 interrupted at intervals by sheaths of 

 sterile leaves, as in some abnormal con- 

 ditions of Equisetum (Fig. 197). Some 

 specimens of Phyllotheca have, however, 

 been described by M. Zeiller as having 

 strobili like those of Annularia, that is, 

 of the Calamostachys-type? The other 

 genus is Schizoneura, of Triassic age, 

 characterised by the whorled leaves being 

 associated in webbed sheaths, which may, 

 however, be slit longitudinally to the base. 

 They thus form leaf-like lobes which stand 

 off at a considerable angle from the axis 

 (Fig. 198). The axis is marked by longi- 

 tudinal grooves, which are continuous 

 longitudinally from internode to internode, 

 thus showing that the leaves of successive 

 whorls did not alternate. The fructifi- 

 cation is unknown. 



Most of the older Equisetal fossils, 

 however, belong to the Calamarian type. 

 These plants were often of dendroid habit, 

 with secondary thickening of the stem, but with a similar primary 

 construction of the shoot to that seen in Equisetum. The leaf-whorls 

 are frequently webbed at the base, though often only slightly, as in 

 Annularia ; but in Asterophyllites, which is traced back to the Devonian 

 period, the leaves appear quite separate, in widely divergent whorls. 



1 Kidston, "On the occurrence of the genus Equisetum, etc.," Annals Mag. Nat. 

 Hist., ix., p. 138, 1892. 



2 Zeiller, Palaeobotanique, p. 164. 



FIG. 195. 



Equisetum maximum, Link. A, the 

 upper part of a fertile axis, with the lower 

 half of the strobilus. Natural size, b the 

 leaf-sheath. n; = annulus. x stalks of spor- 

 angiophores cut off. y = transverse section of 

 axis. j9 = sporangiophores in various posi- 

 tions, slightly enlarged. s/ = stalk. sg 

 sporangia. 5 = enlarged distal end. (After 

 Sachs.) 



