PREFACE. 



In presenting this monograph of the most difficult genus of North 

 American plants I have no excuse 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 1 would like to 

 know more, but to wait ti'l 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 killrr and preservative of section 

 material as it causes the least shrinkage and distortion cf tissue, but it 

 is impossible to preserve any material in anything that will not cause 

 STme shrinkn^e. The celloidin method of infiltration and imbedding 

 has proven the method of most service. Some tissues have been 

 st'Mned but for the most part sections have been left unstained and 

 drawn from just as they are in order to get all detaVs. The method 

 of drawing sections has been to mount sections and to project the 

 images on the paper through a camera vv'^hich magnifies them three 

 times and to trace the image on the drawing paper with a pencil and 

 when all details are co r))]pte 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 puh'ication of the abortive list of Sheldon in 1894 

 convinced me that no one but a fieVl •^^otanist could ever monograph 

 the genus right, and for that reason T notified the young man that 

 whatever work he did on the genus would not deter me from finally 

 nionograi)hing it. 



In getting my material and observations I have traversed the 

 rontinent 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 monograi)h has been in i)reparation. 



I am indebted to nearly every North American Botanist for speci- 

 mens lo;ined or notes taken. Those to whom I am most indebted 

 nre the Brandegses for the loan of all their material and for many 

 notes and s-'eeimens through the years past. Miss Alice Eastwood 

 Joaned me ill the material of the California Academy of Science 



