EXPERIMENTAL INVESTIGATIONS ON BIRCH AND OAK? 
EpitH 8S. WHITAKER 
(WITH PLATES XII-XV AND FOUR FIGURES) 
At the present time experimental investigations have a well 
merited vogue, especially in genetical and morphological fields. 
Until recently, however, little has been done in this connection on 
the structure of woody plants. This has been due largely to the 
fact that the history of woody plants has not been sufficiently 
understood to warrant their interpretation. The doctrine of 
evolution was formulated mainly through the study of comparative 
anatomy in the absence of a fossil record. Plant tissues, however, 
are more resistant to decay than animal tissues (with the exception 
of bones), and as a consequence their historical relations and 
affinities have become available for comparison with existing forms 
and structures. The comparative anatomical study of. existing 
and fossil plants has led to the conclusion that there are certain 
general principles which not only hold true for a group of plants 
in which there is a fossil record, but also may be applied to other 
groups in which, as in the Angiosperms, there is as yet no complete 
geological record. Certain general conclusions thus inductively 
established make it possible to apply the same principles in judging 
anatomical features and interpreting structural relationships in 
the Angiosperms, in absence of fossil record (4, 5, 7). 
The first principle established as a result of comparative 
anatomical study and knowledge of fossil forms, and one employed 
by zodélogists, is based on the fact that in the course of their devel- 
opment organisms may pass through conditions now lost in adult 
life, but once possessed by the organism in its mature state. This 
is called the law of recapitulation, and holds true in plants also. 
For example, certain of the Cupressineae which have small leaves 
in the adult plant, in their seedling development have the larger 
leaves characteristic of the more ancient flora. 
* Contribution from the Laboratories of Plant Morphology of Harvard University. 
Botanical Gazette, vol. 71] [220 
