26 
A. G. Tansley. 
have dealt with the general aspects of my topic. To put the matter 
shortly I am inclined to believe that the fern-leaf is in phylogenetic 
origin a branch, or rather a branch-system, of a primitive undif¬ 
ferentiated sporangium-bearing thallus, which has “dichotomous” 
branching and unlimited growth, and not an appendicular organ 
differing ab initio from the axis on which it is borne. The modern 
statement of this hypothesis is due apparently in the first instance 
to Potonie 1 ; it has been taken up and discussed by Hallier 2 , who 
aptly calls it the “parasynthallium” theory of the origin of the leaf; 
while Lignier 3 has put forward essentially the same hypothesis, 
apparently independently, and has discussed on this basis, in an 
interesting and ingenious manner, the relationship of the Ferns to 
the Sphenophyllales and Equisetales. 
While I am by no means prepared to accept all the views of 
any one of the three authors mentioned, 1 am of opinion that a 
strong case has been made out for the hypothesis as stated above. 
This belief carries with it the necessity of looking upon the 
branching away of the leaf-trace from the vascular system of the 
stem as in origin a “ dichotomy,” i.e. a separation of the vascular 
strand into two more or less equivalent branches, and 1 think that 
the phenomena of leaf-trace origin among some of the more primitive 
Ferns lend support to this view. 
The theoretical basis of an exposition in lecture form is 
necessarily of the nature of a parti pris. If the lecturer is 
constantly halting between two opinions the material becomes spun 
out so as to lose its coherence and a chaotic impression is produced 
upon his hearers. I do not find that what may perhaps be 
considered the more “orthodox” view of the leaf-trace as the 
vascular supply of an originally appendicular organ possesses any 
sufficient theoretical basis from which the morphology of the more 
primitive types of Filicinean leaf-trace can be intelligibly treated, 
and this naturally disposes me to adopt the rival hypothesis. The 
question being one of origins it naturally mainly affects the treatment 
of the more primitive types, and it will probably be generally 
admitted that it is useful to attempt to focus our knowledge of these 
types from a somewhat unaccustomed point of view. 
1 PotoniS. Die Herkunft des Blattes. Deutsche Bot. Monat- 
schrift 15., 1897 ; and numerous other contributions. 
3 Hallier. Beitriige z. Morphogenie der Sporophylle und des 
Trophophylls in Bez. z. Phylogenie der Kormophyten. Jahrb. 
Hamb. Wiss. Anst. 19, 1901. 
“ Lignier. Equisetales et Sphenophyllales. Leur origine 
filicineenne commune. Bull. Soc. Linn. Norm. 5e ser. 7 
Caen, 1903. 
