Bower.—Studies in the Phytogeny of the Filicales. VIP 31 
involution of the endodermis to form the ‘ pocket ’ at the departure of the next higher leaf-trace, 
(x 150.) 
cortex. There is in fact the first indication of a ‘ pocket \ It may be held 
as a natural consequence of just such an exaggeration of the steps seen in 
the lower leaves as would result from an increase of size of the leaf and of 
its trace. The involution gradually smooths out again upwards after the 
leaf-trace has separated off, thus the pocket is shallow. But before the 
level of the next higher leaf-trace is reached, sclerotic cells surrounded by an 
endodermis appear in the centre of the stele, and widening out upwards, 
communicate at the next leaf-insertion with the outer cortex. This is the 
first actually complete foliar gap, and from this point onwards such gaps of 
the usual type occur at each leaf-insertion, the structure being as a conse¬ 
quence solenostelic. 
This ontogeny has been described because it illustrates clearly the 
critical point in that succession of phases of development of the stele which, 
though variable, is widespread in Ferns. In various cases it may be more 
or less curtailed, or lengthened. The stages are (1) Protostele, (2,) medul- 
lated Protostele, (3) Lindsaya- condition, (4) Solenostele. 
