IX] LEAF-SHEATHS OF EQUISETITES. 259 



of coal pressed on to the surface of a sandstone cast, or covering 

 a flattened stem-impression on a piece of shale. It is sometimes 

 possible under the microscope to recognise on the carbonised 

 epidermal tissues the remains of a surface-ornamentation 

 similar to that in recent species, which is due to the occurrence 

 of siliceous patches on the superficial cells. Longitudinal rows 

 of stomata may also be detected under favourable conditions of 

 preservation. The nodal diaphragms of stems have occasionally 

 been preserved apart, but such circular and radially-striated 

 bodies may be misleading if found as isolated objects. Casts 

 of the wide hollow pith of Equisetites, with longitudinal ridges 

 and grooves, and fairly deep nodal constrictions, have often 

 been mistaken for the medullary casts of Calamites. 



Several species of Equisetites have been recorded from the 

 Upper Coal-Measures and overlying Permian rocks, but these 

 present special difficulties. In one instance described below, 

 (Equisetites Hemingwayi Kidst.), the species was founded on a 

 cast of what appeared to be a strobilus made up of sporophylls 

 similar to those in an Equisetiwi cone. In other Permo- 

 Carboniferous species the choice of the generic name Equisetites 

 has been determined by the occurrence of leaf-sheaths either 

 isolated or attached to the node of a stem. The question to 

 consider is, how far may the Equisetum-like leaf-sheath be 

 regarded as a characteristic feature of Equisetites as distinct 

 from Calamites ? In the genus Calamites the leaves are 

 generally described as simple linear leaves arranged in a whorl 

 at the nodes, but not coherent in the form of a sheath (fig. 85). 

 The fusion of the segments into a continuous sheath or collar 

 is regarded as a distinguishing characteristic of Equisetites and 

 Equisetum. The typical leaf-sheath of a recent Horee-tail has 

 already been described. In some species we have fairly large 

 and persistent free teeth on the upper margin of the leaf-sheath, 

 but in other Equisetums the rim of the sheath is practically 

 .straight and has a truncated appearance, the distal ends of tho 

 segments being separated from one another by very slight 

 depressions, as in a portion of the sheath of Equisetum ravw- 

 sissimum Desf of fig. 58, C. In other leaf-sheaths of this 

 .species there are delicate and pointed teeth adherent to the 



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