He first gives in detail all the many points of structure which vary in the several species, and estimates numerically the relative frequency of the variations.

He specifies above a dozen characters which may be found varying even on the same branch, sometimes according to age or development, sometimes without any assignable reason.

Such characters are not of course of specific value, but they are, as Asa Gray has remarked in commenting on this memoir, such as generally enter into specific definitions.