598 E antes. — On the Occurrence of 
trace. Queva, in his study of leaf-development in this species, however, did 
not find it. 
Equisetum shows, therefore, centripetal wood, not only in the traces of 
the sporophylls, but also in those of the vegetative leaves. The descent 
of this genus from ancient Calamites of the type of C. petty cur ensis is thus 
made highly probable. The centripetal wood in the leaf-trace of the 
Equisetales, together with many other characters, seems to indicate a clear 
relation of these plants to the Sphenophyllales, and through them, perhaps, 
to the Lycopodiales, which the latter resemble in anatomy. 
The Lycopodiales and the Sphenophyllales possess centripetal wood in 
their stem structure. In C. petty cur ensis , alone among the Calamites, small 
arcs of centripetal xylem remain in the stem-bundles. The stem proper of 
all other known Equisetales has completely lost it, and it has persisted 
only in the more conservative organs, the reproductive and vegetative leaves. 
The structure of the foliar traces of the Tertiary, and especially of the 
Mesozoic, Equisetales should prove very interesting. Some of the Equise- 
taceous fossils of the latter era have recently been shown 1 to be interesting 
in other respects — in the number and course of the stem-bundles and 
in the markings of the spores. Doubtless, when well-preserved material is 
available, they will show considerable centripetal xylem in their foliar 
traces. In the stems of the living members of this class secondary 
growth has nearly disappeared ; the centrifugal primary wood has largely 
become parenchymatous, and the centripetal has completely vanished. 
Through the countless generations of Equisetaceous plants, from the 
Lower Carboniferous Period to the present day, the vegetative leaf has 
continued to form centripetal wood. This places new and strong emphasis 
upon the conservatism of that portion of the vascular system which sup- 
plies the leaves. Its value in this respect has often been noted, but has 
perhaps not yet received the attention it deserves. A few instances in 
different groups of plants may be cited. The Cycads present a case 
parallel in many respects to that of Equisetum. Mettenius, in i860, dis- 
covered mesarch structure in their leaf-bundles. Dr. Scott 2 has shown that 
the peduncles of several genera have mesarch bundles ; also that the traces 
of the vegetative leaves and, to a less extent, of the sporophylls have centri- 
petal growth. The vegetative stems of the Cycadaceae, however, have 
only centrifugal wood. Yet their probable ancestors among the Pterido- 
spermeae had mesarch bundles in the stem. Among the Cordaitales the 
most ancient forms had mesarch stem and mesarch leaf-bundles ; later 
forms had the same leaf-structure, but endarch structure had become the 
1 Halle, T. G. : Zur Kenntnis der mesozoischen Equisetales Schwedens. Uppsala und Stock- 
holm; 1908. 
2 Scott, D. H. : The Anatomical Characters presented by the Peduncle of the Cycadaceae. Ann. 
Bot., vol. xi ; 1897. 
