999. The diagnoses, or generic and specific characters of plants are the means used by botanists for distinguishing them from each other. These characters are of two sorts: the one is called essential, and is most commonly employed for orders and genera; the other is called differential, and is chiefly used in discriminating species. The differential character conveys information respecting the differences between one thing and another, and the essential character expresses those peculiarities which are known to be essential. ' In constructing essential and differential characters in Latin, it is customary to use the nominative case for genera and orders, and the ablative for species ; but in English the nominative only is employed in both cases.' (Ibid. p. 439.)
