MICHIGAN ACADEMY OF SCIENCE. 131 



habit of the plant, the character of the bark and the absence or presence 

 of spines on the stems and large branches, the nature of the soil and situa- 

 tion where the plant grows, its abundance and such other information as 

 cannot be obtained by the examination of the herbarium specimen. If 

 these simple rules are followed there is no difficulty in preparing specimens 

 that can be used for the determination of species and the description of new 

 ones, and the Academy, through its members living in different parts of the 

 state, can by organizing a systematic study of this interesting genus, make 

 an important contribution to the knowledge of the North American flora. 



Following this introduction is a systematic treatment of the species de- 

 scribed, accompanied by keys for the identification both of the sub-genera 

 and of the species. 



Jamaica Plain, Mass. 



