Ethel N. Thomas. 
77 
A THEORY OF THE DOUBLE LEAF-TRACE FOUNDED 
ON SEEDLING STRUCTURE. 
By Ethel N. Thomas, B.Sc. 
(University College, London). 
[Text-Figs. 1 — 4.] 
VER since the principle of recapitulation of the history of the 
race in the development of the individual became fully 
realized by biologists, and summed up in the well-known phrase 
“ Ontogeny repeats Phylogeny,” Botanists have endeavoured to 
use this instrument for disclosing the history of the ages, in the 
hope that it might prove as fruitful for them as it has been for 
Zoologists. 
These expectations were, however, not fully realized, and the 
early attempts may be said to have resulted largely in disappoint¬ 
ment. This is in a great measure due to the fact that the seriation 
of developmental stages in plants and in animals respectively is so 
little comparable, that great uncertainty arises as to what portion 
of the life-history of a plant should be included in the study of its 
embryology. Thus in addition to the more restricted aspects of 
embryology, great enthusiasm was shown for many years in the 
study of apices, meristems, and all growing regions, even of the 
later formed organs. 
The early embryologists proper confined themselves to the 
study of the development of the ovum and of the differentiation 
from it of the first-formed members of the young plant. The 
minute comparative study of the early cell-divisions revealed so 
much inconstancy and variation that so far it has rendered little aid 
to phylogeny. 
At the meeting of the British Association held in Liverpool in 
1896, Dr. Scott drew attention to the lack of sustained effort in this 
direction, and also to the comparative neglect of the study of the 
later stages of development, during which organs and tissues are 
being differentiated. Since that time, i.e., during the last ten years, 
much has been done to remove this reproach, and a new school of 
embryologists has arisen. In the Cryptogamic field, the work of 
Professor Jeffrey, Mr. Gwynne-Vaughan, Mr. Boodle, Mr. Tansley 
and Miss Lulham, and of Dr. Chandler has yielded interesting and 
important results. 
The plant embryo may be said to superpose its stages of 
development in space rather than in time, producing more and more 
