1915] PLOWMAN—BOX ELDER 189 
decurrens Lesq. (Colorado), N. triloba Newberry (Ft. Union Beds, 
North Dakota); OLicocENE, N. bohemica Menzel (Bohemia); 
Miocene, JN. trifoliata Braun (Oeningen in Baden), A. Negundo 
(from Oeningen, 1835; doubtful). To these may be added the 
apparent box elder described by KNowLtTon, under the name of 
Rulac crataegifolium, from the Miocene of the John Day Basin, 
Oregon (Bull. U.S. Geol. Surv. no. 204. p. 77. pl. 16. fig. 7). 
Since the Glacial Period the Negundo type has been abundant 
and varied throughout all of the morainal regions. The remark- 
able elasticity of the type under shifting stress of environment 
doubtless accounts in large measure for its unusual success. 
Theoretical considerations 
As a result of the studies and observations outlined in this 
paper, the writer is of the opinion that the box elder, in its present 
highly specialized form, is a product of the Glacial Period. The 
evidence may be stated concisely as follows: 
1. Negundo characters were but slightly developed before the 
Pleistocene or Glacial Period. They have been widespread since 
that time. Negundo occurs in greatest abundance in regions of the 
richest glacial drift, especially upon and below the great terminal 
moraines. 
2. Negundo characters were apparently developed rapidly, and 
partially fixed, through exposure to the inclement conditions along 
the margins of the great continental ice sheet. 
3. Negundo was apparently a primitive variant from the 
ancestral Acer stock, possessing peculiarities especially adapted to 
glacial conditions. These features were greatly emphasized by 
the glacial experience of the species. The impetus gained from 
glacial influences is not yet lost. Negundo is highly variable, yet 
irretrievably separated from the true maples. The nearest points 
of correspondence are found in A. pennsylvanica, A. spicatum, and 
A. platanoides. 
4. Characters of Negundo that would fit it for glacial environ- 
ment are as follows: (a) leaf morphology and anatomy; maximum 
utilization of light; (0) medullary strands in petiole; great capacity 
for transportation; (c) extended insertion of leaf trace into stele; 
