meristems of Ferns as a Phylogenetic Study. 373 
originate deeply in the tissue of the leaf, and are sessile and 
massive, or are even coherent together in masses when mature. 
Finally, in place of the filamentous, Alga-like oophyte, we find 
the fleshy and massive prothallus with its antheridia and 
archegonia not projecting and exposed, but protected from 
drying up by being sunk in the tissue. Every one of these 
characters points in the same direction : and if we once grant 
that the Hymenophyllaceae are phylogenetically the lowest 
members of the Filicineous series (a view which is held by all 
recent writers), then the Ferns illustrate in a most remarkable 
way the emergence of this branch of the Vascular Cryptogams 
from a semi-aquatic to a truly aerial habit. This is to me the 
real meaning of the lowering of the centre of construction, and 
consequent increase of robustness in all these meristems as we 
pass from the Hymenophyllaceae to the Marattiaceae : in it I 
believe that we see the expression of the fact that the higher 
plants of this series, by their more bulky habit, are the better 
adapted to an existence under circumstances of exposure to air 
which is not saturated with moisture. Regarding the series of 
green plants as a whole, those lowest in the scale are character- 
istically dependent upon water for their life, and especially for 
their fertilisation, and in this latter respect at least they betray 
their aquatic origin : in vascular plants it is seen that those 
which we regard as the higher in the scale are as a whole 
those best adapted to living in the air and in comparatively 
dry situations, and they are independent of external water 
as regards their fertilisation 1 . Among the Vascular Crypto- 
gams, however, we see a series in a transition state, emerging 
from the semi- aquatic habit : the differentiation and con- 
struction of the vegetative organs of the sporophyte here 
shows a general similarity to that of the highest forms, but 
the sexual process, which is still dependent for its completion 
upon external water, is a clear index of their position lower in 
1 It is hardly necessary to point out that the aquatic Phanerogams are not. 
typical Phanerogams, but that their simplicity of structure is to be regarded as 
a degradation : their fertilisation is, even in so extreme a case as Va/lisneria, 
characteristically that of an aerial habit, i. e. siphonogamic. 
CC2 
