258 OHIO STATE ACADEMY OF SCIENCE. 
which we use in the descriptions vary enormously, one would still 
recognize without much difficulty that the southern plant was 
Salix nigra in a metamorphosed condition. Consequently, as 
far as the native species are concerned, at least, though it is hoped 
that this paper will be of service all over the northeastern portion 
of the United States, one must expect to find it more and more 
at fault as one recedes from Columbus. Some day when there 
is a monograph like the present covering every state in the coun- 
try, some genius will write a general treatment that will effect- 
ively handle all the species. 
One reason why it is necessary at present to limit the treat- 
ment to a small area is that no one has yet succeeded in describ- 
ing the qualities by which we recognize a willow. We define the 
species by external, artificial characters such as the shape and 
hairness of the leaves, the length of the pedicels and so on; but 
all such characters may vary enormously and still leave the qual- 
_ ities by which the expert determines the species, constant. We 
cannot study willows profitably by tables of dimensions and 
geometrical descriptions of the leaves any more than we can 
study the faces of our friends by measuring the length of their 
noses. As there is a certain almost indefinable individuality in 
a human face, there is in a species of plants an individuality no 
less indefinable but no less important. 
I here desire to extend my thanks to friends who have aided 
me very greatly in the preparation of this paper: to Dr. W. A. 
Kellerman of the Ohio State University, who at one time planned 
to be joint author with me, for innumerable kindnesses done and 
suggestions given during the whole course of the work; to Prof. 
John H. Schaffner of the same institution for help especially in 
verifying and testing the keys and to Mr. Otto E. Jennings of the 
Carnegie Museum for the loan of the collections of that insti- 
tution. : 
The present paper is based on studies and collections of wil- 
lows begun in the spring of 1898 and carried on as opportunity 
afforded ever since. During that time the author has been 
enabled to study the plants in the field in various parts of Ohio, 
in the vicinity of Washington, D. C., and in the Red River Valley 
of North Dakota and Minnesota. The herbarium on which he has 
mostly depended is that of the Ohio State University at Colum- 
bus. The United States National Herbarium and the Herba- 
rium of the Carnegie Museum at Pittsburg have also contributed 
considerably to whatever value the work may possess. 
