TO 



JOHN TORREY, LL.D. 



Almost twenty years have passed since the first edition of this 

 work was dedicated to you, — more than thirty, since, as your pupil, 

 I began to enjoy the advantage of being associated with you in 

 botanical pursuits, and in a lasting friendship. The flow of time 

 has only deepened the sense of gratitude due to you from your 

 attached friend, 



ASA GRAY. 



Cambridge, May 30, 1867. 



