It will be seen that the Key directs the inquirer to ascertain, 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 characters, and, finally, a series of successively subordinated propositions,--each set more indented upon the page than the preceding,--leads to the name of the Order sought for, followed 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. SYNOPSIS OF THE ORDERS OF PLANT DESCRIBED IN THIS WORK. SERIES I. PHÆNOGAMOUS OR FLOWERING PLANTS: those producing real flowers and seeds. CLASS I. DICOTYLEDONOUS OR EXOGENOUS PLANTS. Stems formed of bark, wood, and pith; the wood forming a zone between the other two, and increasing, when the stem continues from year to year, by the annual addition of a new layer to the outside, next the bark. Leaves netted-veined. Embryo with a pair of opposite cotyledons, or in Subclass II. often three or more in a whorl. Parts of the flower mostly in fours or fives. SUBCLASS I. ANGIOSPERMÆ. Pistil consisting of a closed ovary which contains the ovules and becomes the fruit. Cotyledons only two. DIVISION I. POLYPETALOUS: the calyx and corolla both present; the latter of _separate_ petals. (Apetalous flowers occur in various Orders, as noted under the subdivisions.) [A.] THALAMIFLORÆ. Stamens and petals hypogynous (free both from the calyx and from the superior ovary), upon a usually narrow receptacle (not glandular nor discoid, except in Reseda, sometimes stipe-like). (Stamens and petals upon the partly inferior ovary in some Nymphæaceæ.) Apetalous flowers occur in the Ranunculaceæ and Caryophyllaceæ. [*] 1. Carpels solitary or distinct (or coherent in Magnoliaceæ); sepals and petals deciduous (except in Nymphæaceæ); leaves alternate or radical, without stipules (sometimes opposite or whorled and rarely stipular in Ranunculaceæ); embryo (except in Nelumbo) small, in fleshy albumen.