PREFACE. 
In presenting this monograph of the most difficult genus of North 
American plants I have no excuss to offer, though many could be 
made. I know its shortcomings better than anyone else can know. 
There are Mexican species and a few others cf which I would like to 
know more, but to wait til we know all about all species would 
mean never publishing at all. I have tried to study every species in 
the field to get my knowledge first hand. I have used every means 
possible to get and keep fresh material for study. For the most part four 
per cent formaline has been the killer and preservative of section 
material as it causes the least shrinkage and distortion of tissue, but it 
is impossible to preserve any material in anything that will not cause 
some shrinkage. The celloidin method of infiltration and imbeddinz 
has proven the method of most service. Some tissues have been 
stained but for the most part sections have been left unstained and 
drawn from just as they are in order to get all details. The method 
cf drawing sections has been to mount sections and to project the 
images on the paper through a camera which magnifies them three 
times and to trace the image on the drawing paper with a pencil and | 
when all details are corrplete to retrace them in ink. Most of the pods, 
leaves and flowers were drawn by the use of proportional dividers, 
a few were drawn towards the last by the camera method. 
I had done a considerable amount of work on the genus previous 
to 1894, but the pub'ieation of the abortive list of Sheldon in 1894 
convinced me that no one but a fie'd botanist could ever monograph 
the genus right. and for that reason J notified the young man that 
whatever work be did on the genus would not deter me from finally 
monographing it. , 
In getting my material and observations I have traversed the 
Continent twice east and west and examined nearly all types in this 
country, and have gone from Central Mexico to the British line 
several times:and have collected a large amount of material, in the 
twenty-five years that this monograph has been in preparation 
T am indebted to nearly every North American Botanist for speci- 
| mens loaned or notes taken. Those to whom I am most indebted 
. are the Brandegees for the loan of all their material and for many - 
notes and svecimens through the years past. Miss Alice Eastwood  — 
josmed, me cli the material of the California Academy of Science | 
