mer is terns of Ferns as a Phylogenetic Study. 367 
each h Passing upwards through the series this simplicity and 
regularity is gradually departed from in each and all of these 
meristems ; the structure becomes more complex, the identity 
of the single initial is lost, and the regular sequence of seg- 
mentations becomes less definite: this is associated in each 
case with a sinking of the centre of construction.; The first- 
named divisions of the Filicineous series have a typically more 
delicate structure of their mature organs than those named 
later in the series, and this character comes out clearly also in 
the construction of the meristems. Taking into consideration 
for the moment the relative complexity of their meristems 
alone, the above list may be accepted as representing a natural 
series which graduates by gentle steps from the simpler to 
the more complex. 
It may at first sight be felt that to use as a basis for 
arrangement of a series the detailed characters of the meri- 
stematic tissues, is to proceed on insufficient ground, and to 
approach too nearly to that cellular morphology which, if 
pursued universally and consistently into detail, would lead 
to unsound results ; and those who are disposed to object 
will doubtless point out that there is not always a paral- 
lelism between all the meristematic tissues of a given plant. 
Thus in typically eusporangiate forms, such as Ophioglossum 
or Equisetum , a single initial cell with definite segmentation 
is found in stem and root : these exceptional cases will be 
again referred to below. In support of the main conclusion 
of this paper, however, it will be sufficient to point out that on 
the ground of comparative study of the external form and 
internal structure of both the sporophyte and the oophyte, the 
relative positions of certain families is already agreed upon : 
the main series of Ferns is recognised by recent writers as 
extending between two extreme types— the Hymenophyl- 
laceae, which are considered to be of Algal-Bryophytic affinity, 
1 The results of the detailed examination Onoclea Struthiopteris by D. H. Camp- 
bell (The development of the Ostrich Fern ; Mem. of the Boston Soc. of Nat. Hist. 
Vol. IV. No. II) fall in for the most part with those of other observers. I regret 
that this memoir only came to my hands after these pages had been set up in type. 
