IX] ANATOMY OF EQUISETUM. 251 



Thus the vascular strands of each internode alternate in position 

 with those of the next internode. 



There are certain points connected with the vascular bundles 

 in the nodal region of a shoot, which have an important bearing 

 on the structure of fossil equisetaceous stems. Fig. .54 B 

 represents a diagrammatic longitudinal section through the 

 node of a rhizome of Equisetum arvense from which a root h is 

 passing off in a downward direction, and a branch in an upward 

 direction. The black band c in the parent stem shows the 

 position of the vascular strands ; in the region of the node the 

 vascular tissue attains a considerable thickness, as seen at d in 

 the figure. The bands passing out to the left from d go to 

 supply the branch and root respectively. The increased breadth 

 of the xylem strands at the node is due to the intercalation of a 

 number of short tracheids. Fig. 55, 4 shows a transverse section 

 through a mature node of Equisetum maadmum ; pac marks 

 the position of the protoxylem and e that of the endodermis. 

 On comparing this section with that of the internodal vascular 

 bundle in fig. 52, D, the much greater development of wood in 

 the former is obvious; the carinal canal of the internodal bundle 

 is absent in the section through a node. The disposition of the 

 xylem tracheids in fig. 55, 4 shows a certain regularity which, 

 though not very well marked, suggests the development of wood 

 elements as the result of cambial activity. Longitudinal sections 

 through the nodal region demonstrate the existence of "cells 

 similar to those of an ordinary cambium, and a cell-formation 

 resulting from their division which is similar to that in an 

 ordinary secondary thickening."' The short tracheids which 

 make up this nodal mass of xylem differ from those in the 

 internodal bundle in their smaller size, and in being reticulately 

 thickened. There is, therefore, evidence that in the nodes of 

 some Equisetum stems additional xylem elements are produced by 

 a method of growth comparable with the cambial activity which 

 brings about the growth in thickness of a forest-tree^- The 



1 Cormaok (93) p. 71. 



2 Williamson and Scott (94) p. 877. These authors, in referring to Cormaok's 

 description of the secondary nodal wood of E. maximum, express doubts as to 

 the existence of such secondary growth in all species of the genus. 



