378 Bower . — The comparative examination of the 
the simpler forms, while in the Ophioglossaceae it is rather 
the root and stem which retain the simpler construction. This 
would therefore warn us of the danger of applying too closely 
in detail the correlation of exposed habit with increased 
meristematic complexity. 
Again, in Equisetum , which is eusporangiate, the roots and 
stems have a single initial, with diagrammatically regular 
segmentation: it is to be noted that in these plants under- 
ground rhizomes are the rule, and it is only in their later 
stages of growth that the stems arise above ground. In many 
species of Selaginella , on the other hand, a single initial cell is 
formed in the sub-aerial stem ; in other species there is a more 
complex structure of the meristem, while in Lycopodium three 
initials have been observed, though the general habit is similar 
to that of the allied Selaginellas. These notes will suffice to 
show that speaking of the Vascular Cryptogams as a whole, 
we are not in a position to state that there is any strict, in- 
variable, and definite correlation of structure of meristems 
with exposed habit, or that there is any exact parallelism of 
progression in complexity in all the members of a given 
individual, species, or family. Still, I do not think that such 
exceptions as those above quoted invalidate my general con- 
clusion that in the Filicineous series, taken as a whole, the 
progression of complexity of meristematic structure is to be 
taken as one index among others of the rise of the series from 
a semi-aquatic to a more distinctively sub-aerial habit, a 
conclusion which is borne out by comparison of the texture of 
the mature parts, and of the oophytes and sexual organs. 
The texture of the leaves of the Ferns above studied, and 
their wings, present characters which deserve further remark, 
and especially we may draw a comparison between the 
‘ filmy ’ Ferns, and their more robust neighbours. It has 
been repeatedly pointed out that the characters of submerged 
leaves of the higher plants, as distinct from subaerial leaves, 
are that they are thin, being composed of only few layers of 
cells ; that stomata are absent ; that there is no distinction of 
palisade and spongy parenchyma ; and that the chlorophyll is 
