HOOKER LECTURE, 1922. 223 
correctness of this general principle: migrants to America and the Far East 
still survive, while many of those which followed a more direct southerly 
route perished by the way or were unable to reach the warmer regions in the 
South. At first sight the past distribution of Onoclea would seem to fall into 
line with the wanderings of later Tertiary plants ; but if the American fossil 
Onocleas are correctly referred to an Upper Cretaceous horizon, while those 
from Greenland and Mull are Tertiary in age, some other interpretation must 
be sought. It is important, therefore, to review the evidence more fully than 
is possible in this lecture, and to enquire further into the relative age of the 
beds in which Onoclea has been discovered. 
GLEICHENIACEE. 
Though in the main the nomenclature of Christensen’s invaluable * Index 
Filieum ' is followed, an exception is made in assigning generic rank to the 
tropical Australian Fern, Platyzoma microphylla, which the Danish author 
regards as a subgenus of Gleichenia: this departure from authority is, 
I venture to think, justified by the recent work of Prof. Thompson, of 
Liverpool, The genus Gleichenia includes two subgenera, Æugleichenia and 
Dicranopteris : the latter designation is, however, used by Underwood in 
place of the more familiar Mertensia as a title of generic rank equivalent to 
Gleichenia. Gleichenia in the wider sense includes over 100 species, the 
great majority of which are characterised by the possession of dichotomously 
branched fronds endowed with a power of unlimited growth. Though 
Goebel objects to the common practice of describing Gleichenia fronds as 
dichotomous, it is generally agreed that the method of branching is a form of 
dichotomy, and, as Dr. Boodle has suggested, the pinnate habit of the great 
majority of Ferns is probably a derivative of a more primitive dichotomous 
system. There are few Ferns which it is easy to recognise by frond habit 
alone. The West Indian Bramble Fern (Odontosoria aculeata) has dichoto- 
mously branched fronds, but the pinnules are entirely different from those of 
Gleichenia. With the exception of Matonia pectinata and M. Foxworthyi 
there are no recent Ferns which, without reference to soral characters, could 
be mistaken for Gleichenia. The strong rachis with its periods of rapid 
elongation and inactivity may reach a height of over 20 feet, giving off in 
regular stories forked pinne bearing linear segments usually 3 to 4 cm. long, 
but in rare cases as long as 10 cm. and more. As Underwood says, 
herbarium specimens give a very inadequate idea of the habit and size of the 
splendid fronds of this genus. The linear form of segment with the sori 
either in the middle or in the fork of the lateral veins is one of the distin- 
guishing features of the subgenus Dicranopteris. In Eugleichenia the 
segments are for the most part semicircular, 2 to 3 mm. broad, and the sori 
are borne on the ends of lateral veins. The minute segments of some 
species look like small crenulations on the axes of the pinnæ, and when the 
