viu. PREFACE. 



obvious to the practiced eye of the experienced col- 

 lector. 



Among these is the particular thing which I wish 

 to direct attention to here, viz. : the disarrangement 

 of the species in the genera. It will be observed 

 that while the genera have been arranged in their 

 proper natural order, the species are often grouped 

 in the text quite otherwise. The reason is, I have 

 taken those species, in genera which contain several 

 for treatment first, which, on account of their common- 

 ness, or peculiar habitat or appearance, could be most 

 easily and certainly identified. From these I have 

 proceeded step by step to the more difficult plants. 



Then again I have often found it convenient to 

 group certain species together for the advantage of 

 comparison in the description which do not always 

 naturally belong together. You will therefore under- 

 stand that, while the orders and genera follow their 

 natural grouping, in the text, the species in the 

 genera cannot be depended upon to do so, in most 

 caaes. 



I must add a single remark further on this general 

 subject. While the several sub-classes, the Green, 



