The Botryopteridece , 67 
This specialisation and partial isolation of conducting channels 
is a very general result of the progressive evolution brought about 
by increasing demands for conduction along certain lines. It may 
be traced for instance in the evolution of the stem-stele of Zygopteris , 
assuming, as seems probable, that it arose from a stele of the 
Botryopteris- type. The lobes of the stele represent the beginning 
of specialisation of leaf-traces, and this is combined with the 
of a. These figures should be turned through a right angle to correspond 
in orientation with Figs. 8—11. Univ. Coll. Collection, K 36a, b, c. x 4. 
enlargement of the periphery of the stele and the failure to develop 
many of the central tracheids, which become no longer necessary. 
This tendency is carried further in the genus Anachoropteris 
(Fig. 20) where the lobes of the stele become more prominent. In 
Fig. 20. Anachoropteris Decaisnii. Stem stele with star-shaped xylem (x) 
and pith (p), extending into the arms of the star. 
both genera the stele-lobe tends to become double (Figs. 6, 7, 20). 
This is probably to be interpreted as a basipetal development into 
the stem of the biseriate insertions of pinna-traces on the rachis- 
