THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 275 



spiral protoxylem region flanked inwardly and outwardly by 

 reticulate metaxylem. The condition present is, in fact, mesarch 

 and strikingly resembles the common anatomical situation in the 

 leaf of the Lycopodiales as described in an earlier chapter. In 

 the sporophyll of the living representatives of the Equisetales the 

 mesarch structure of the trace is even more conspicuous than in 

 the vegetative leaf. The situation in this respect is shown for 

 E. palustre (b}. But the reproductive leaf not only manifests cen- 

 tripetal wood in its trace, but 

 it also presents an equally 

 significant condition in the rela- 

 tion of the phloem to the xylem. 

 In c is shown a transverse sec- 

 tion of the trace of the sporo- 

 phyll in 'E. hi em ale. Sieve 

 tubes can plainly be seen sur- 

 rounding the xylem elements, 

 while the latter have in their 

 midst a more or less obvious 

 lacuna representing the evanes- 

 cent protoxylem. On the 

 grounds of comparative anat- 

 omy it is clear that the 



FIG. 198. Transverse section of the 



Equisetaceae once possessed roo t of Equisetum hiemale. 

 centripetal wood in the stem. 



That this was the former situation for the stock as a whole is clearly 

 indicated by the anatomy of Protocalamites shown in Fig. 190. It 

 is further rendered highly probable by the concentric as well as the 

 mesarch organization of the trace of the sporophyll in the living 

 genus that the bundles of the axis were formerly concentric in their 

 organization. This condition must, however, have been realized in 

 the extremely remote past, as no indication of concentric structure 

 has been found in the stems of even the most ancient anatomically 

 investigated remains of calamitean forms. 



The root in the genus Equisetum (Fig. 198) shows the presence 

 of four protoxylem groups alternating with as many clusters of 

 elements of the phloem. The metaxylem consists ordinarily of a 



