i^o A. G. Tansley. 
external at the insertion of the trace, but usually the leaf-trace joins 
the stele in the form of a closed ring, and no connexion between 
the internal and external tissues takes place. In G.flabellata (Fig. 
40) the internal phloem, the internal endodermis and the scleren- 
chyma in the concavity of the C-shaped leaf-trace are continued 
down into the xylem of the stele below the node to form a bulky 
pocket. The sclerenchyma and endodermis first die out, leaving 
the internal phloem, which eventually also dwindles and disappears. 
A B 
Fig. 38. Gleichenia circinata. 
A. T. S., leaf-trace. 
B. Junction of leaf-trace with stele, e.-e.se., extra-cndodermal 
sclerenchyma; i.-e.se., intra-endodermal sclerenchyma. 
G. pectinata (Fig. 41) is a solenostelic form, with a normal 
centrally placed sclerenchymatous pith in the internodes, sur¬ 
rounded by endodermis and internal phloem. A bulky “nodal island” 
is formed in connexion with the junction of the large C-shaped leaf- 
trace with the stele. This connects with the pith at the node, but 
is sometimes, at least, continued for some distance down into the 
internode in a dorsal position, independently of the pith (Fig. 41C). 
Finally, Platyzoma has a curious and rare type of stele (Fig. 
42), containing a pith surrounded by an endodermis but no internal 
phloem, and with quite small collateral leaf-traces, whose departure 
does not break the continuity of the vascular ring as it does in the 
normal solenostelic type. 
It is obvious that the types described fall into a rough series 
extending from the simpler forms of Engleichenia on the one hand 
through the types with internal endodermis at the base of the 
petiole only, and then through G. linearis, to the typical Mertensias 
and finally to G. pectinata. It is scarcely possible, however, to 
regard this as a progressive evolutionary series, in spite of the fact 
that the simpler forms have a certain resemblance to many of the 
