No. 461.] AFFINITIES OF EQUISETUM. 283 
first. The oldest fossils show the same type, and in the embryo, 
before any trace of the vascular bundles can be seen, there is 
developed the three-leaved primary foliar sheath, and corre- 
sponding to the three teeth of the foliar sheath, are developed 
simultaneously the three vascular strands of the first internode. 
THE SPHENOPHYLLALES. 
The Sphenophyllales are exclusively fossil forms which show 
evident affinities with the Equisetales, from which they differ 
especially in the arrangement of the tissues of the stem. 
Instead of the large central cavity found in the stem of Equi- 
setum, Sphenophyllum has an axial solid vascular cylinder more 
like that of the modern lycopodiaceous forms than like any 
other existing pteridophytes. 
Scott! considers that the Sphenophyllales are to some extent 
intermediate between Equisetales and Lycopodiales, basing his 
opinion mainly upon the anatomical characters, especially those 
of the stem. The peculiar fossil Cheirostrobus? he especially 
considers a synthetic type, having the sporangia of the Equi- 
setum type, but the vascular system of a lycopod. 
Potonie? holds that the Sphenophyllales are an early offshoot 
of the equisetaceous series, perhaps directly connected with 
Archzocalamites. The characteristic triarch or hexarch bundle 
of the stem suggests a three-sided apical cell like that of Equise- 
tum, but in the Psilotaceze, which are commonly associated with 
the Lycopodiales, there is a somewhat similar type of stele, and 
these forms also have a single tetrahedral initial cell in the 
shoot. 
One objection to considering the Sphenophyllales a synthetic 
type is that all the existing types of pteridophytes were clearly 
differentiated in formations earlier than any in which remains of 
the Sphenophyllales certainly occur. 
1 Studies in Fossil Botany, 1900. 
2 Joc. cit. pp. 106-114. 
3 Joe. cit, p. 180. © 
