576 Humphreys : An analogy 



entire leaf as the earliest ancestral form, the fossil representatives 

 of which are at present either undiscovered or perhaps incorrectly 

 referred to certain other lauraceous genera. 



Finally, it appears logical to infer in regard to reversionary 

 leaf forms, which are really not reversions but forms which have 

 not developed beyond the ancestral condition, that if they do occur 

 they should appear when the growth of the tree is least vigorous, 

 and that the leaves of full development should appear when the 

 tree is in its period of maximum growth. Growth is least vigor- 

 ous in early spring, when the tree is awakening from its dormant 

 condition, and in autumn when it is preparing for it. The leaves 

 which appear at these periods are the simplest in form. Summer 

 is the time of maximum vigor of growth and the leaves which ap- 

 pear at that period are the most highly specialized or differentiated. 

 Further than this, in any given growing season the bud for the 

 next season is already being formed. The outer leaves of the bud, 

 which become the lower leaves of the next season's branch, are 

 the first formed and are therefore formed during the early part oi 

 the growing season, while the innermost ones, which subsequently 

 become the uppermost ones of the future branch, are formed late 

 in the growing season. These are the simplest in form. The in- 

 termediate zone of bud leaves, which are destined to occupy the 

 median part of the branch, are formed during the height of the 

 growing season, when conditions are most favorable for full devel- 

 opment. These are the lobed ones. 



From every point of view in which actual facts are in our pos- 

 session it therefore seems reasonable to regard the entire leaves as 

 the type of the ancestral form, and the arrangement of the seveia 

 leaf forms on the branch as representing the development of the 

 organism as a whole from infancy to maturity and back again to 

 second childhood or old age. 



I desire to acknowledge the kindly assistance of Professor . 

 W. Grabau, of Columbia University, and of Dr. Arthur Hollick, o 

 the New York Botanical Garden, in the preparation of this paper- 





