vm PREFACE. 



specific characters or even a synoptical table of his plants. 

 Without either of these helps it is exceedingly difficult to 

 identify them. Fortunately specimens of a considerable 

 number of them are published in Wirtgen's excellent Herb. 

 Ruhorum Rheaanorum, and many others are contained in 

 Mr J. G. Baker's extensive Herbarium, which he most 

 kindly placed for a lengthened period in my hands. 



Cambridge, 



May r, 1869. 



