PTERIDOPHYTA 



49 



(i) Calamites (Figs. 13, 150). — A tree often attaining a height 

 of one hundred feet, usually preserved in the form of casts of the 

 interior of the hollow stem. The markings on the surface of 



Fig. 13. — ^ The ancient horsetail. A, B, Calamites stickowi Brongniart, from the 

 Carboniferous coal deposits of Pennsylvania. A, a cast of the hollow interior of 

 a small section of the stem ( X f). B, a. portion of the surface of this in detail. 

 C, diagrammatic cross section of a mature calamite stem to show origin of the 

 ribs and grooves upon the cast, gr., grooves (internodal furrows) formed by the 

 inwardly projecting ends of the vascular bundles; i.ca., casts of infranodal canals 

 opening in the medullary raj's at the nodes; m.r., medullary rays; «., nodes; 

 r., ribs (internodal ridges) formed by the groove upon the inner surface of each 

 medullary ray ; i'.Z>., vascular bundles. {A, B from Lesquereux.) 



these casts are the print of the inner surface of the wood and do 

 not correspond to the ribbing of the external surface of an 

 Equisetum. Hence conclusions establishing the relationship 

 of Calamites to the horsetails cannot be based on the external 



