4 PREFACE. 



Analytical Key to the Orders. — As stated in Dr. Gray's 

 Preface to the last edition, this is designed to enable the stu- 

 dent to refer readily to its proper Order any of our plants, 

 upon taking the pains to ascertain the structure of its flowers, 

 and sometimes of the fruit, and by following out a series of 

 easy steps in the analysis. It is founded upon the most ob- 

 vious distinctions which will answer the purpose, and is so 

 contrived as to provide for all or nearly all exceptional in- 

 stances and variant cases. Kef erring to the Order which the 

 Key leads him to, the student will find its most distinctive 

 points brought together and printed in Italics in the first sen- 

 tence of the ordinal description, and thus can verify his results. 

 The Synopsis which Ic;, ows will then lead him to the genus, 

 to be verified in turn by the full generic description in its 

 place; and the progress thence to the species is facilitated, 

 when there are several to choose from, by the arrangement 

 under divisions and subdivisions, as already explained. 



It will be seen that the Key directs the inquirer to ascer- 

 tain, first, the Class of the plant under consideration, — which, 

 even without the seeds, is revealed at once by the plan of the 

 stem, as seen in a cross-section, and usually by the veining of 

 the leaves, and is commonly confirmed by the numerical plan 

 of the flower ; — then, if of the first Class, the Subclass is at 

 once determined by the pistil, whether of the ordinary kind, 

 or an open scale bearing naked ovules. If the former, then 

 the choice between the three Divisions is determined by the 

 presence or absence of the petals, and whether separate or 

 united. Each Division is subdivided by equally obvious char- 

 acters, and, finally, a series of successively subordinated prop- 

 ositions, — each set more indented upon the page than the 

 preceding, — leads to the name of the Order sought for, fol- 

 lowed by the number of the page upon which it is described 

 in the body of the work. 



The book is now submitted to those for whose benefit it has 

 been prepared, in the trust that its shortcomings will meet 

 with friendly indulgence, and with the earnest request that 

 information be kindly given of any corrections or additions that 

 may appear to be necessary. 



SERENO WATSON. 



Cambridge, Mass., Dec. 26, 1889. 



