272 THE ANATOMY OF WOODY PLANTS 



endodermis except in the region of the nodes where continuous 

 internal and external endodermal layers are seen. The pith of 

 the genus Equisetum, particularly in the region of the nodes, is 

 frequently characterized by the presence of nests of dark-brown 

 sclerotic cells, resembling similar structures found in the cortical 

 tissues of both stem and leaf. On the grounds of comparative 

 anatomy we shall accordingly be compelled to regard the pith 

 of Equisetum as of cortical origin. 



The arrangement of the fibrovascular strands at the nodes in 

 living and fossil representatives of the Equisetales (Equisetaceae 

 and Calamitaceae) must now be considered. The situation present 

 is best revealed by means of diagrams (Fig. 196). In A is depicted 

 the arrangement of the fibrovascular structures at the node in the 

 vegetative stem of the living Equisetum. Across the center of the 

 diagram passes a heavy transverse band, the so-called nodal wood. 

 In this the strands of the upper and lower internodes end in such a 

 manner that they alternate with one another. The traces of the 

 leaves originate from the strands of the lower internode and thus 

 subtend the intervals between the strands which are joined with 

 the nodal wood from above. A superficial view of the topograph- 

 ical conditions represented here would result in the conclusion that 

 the Equisetaceae are provided with foliar gaps precisely as is the 

 case in the Pteropsida. A consideration of B makes this view of 

 the matter difficult to sustain. In the figure the foliar strand is 

 represented in radial aspect as it comes off from the fibrovascular 

 tissues of the axis. It is clear that the trace of the leaf takes its 

 origin below the so-called nodal wood and passes out without 

 showing any foliar gap above it. It is true that, as is indicated in 

 both A and B, a gap is present subtending the foliar trace above 

 the continuous zone of the wood at the node; but a consideration 

 of the historical and comparative anatomical data makes it difficult 

 indeed to regard the gap in question as a foliar one. In C is shown 

 the arrangement of the strands of the primary wood in an ancient 

 calami tean form (Archaeocalamites) . It is evident in this case 

 that the strands of the upper and lower internodes, instead of 

 meeting the nodal wood in alternation as they typically do in 

 Equisetum, exactly coincide with one another and are not subject 



