XXIV. CRYPTOGAMIA: Ferns, mosses, lichens, sea-weeds, mushrooms, &c. The orders or subdivisions of the classes are founded on the number of the pistils in the first thirteen. Thus, in any of these classes, the first order is _Monogynia_, or one pistil; the second _Digynia_, two pistils, &c. But in the fourteenth class, Didynamia, there are only two orders, _Gymnospermia_ and _Angiospermia_, the former having four naked seeds, the latter having the seeds enclosed in a seed-vessel. In the fifteenth class, Tetradynamia, there are also two orders, _Siliculosa_, in which the pod is short, and _Siliquosa_, in which it is long. The orders of Monadelphia, Diadelphia, and Polyadelphia, are formed from the number of the stamina, and bear the names of _Hexandria_ when there are six, _Decandria_ when there are ten, &c. The orders of the nineteenth class, Syngenesia, are six. In the first, _Polygamia aequalis_, all the flowers (or _florets_, as they are here called on account of their small size, and because they are viewed as components of a compound flower) have stamens and pistils, and are equally fertile; in the second, _Polygamia superflua_, the flowers of the centre have stamens and pistils, those of the circumference pistils only, but both kinds produce seeds; in the third, _Polygamia frustranea_, the flowers of the centre have stamens and a pistil, and are fertile, those of the circumference neutral, or furnished with a pistil, but steril; in the fourth, _Polygamia necessaria_, the flowers of the centre have stamens and a pistil, but are steril, in consequence of an imperfection in the stigma, those of the circumference have a pistil, and are fertile; in the fifth, _Polygamia segregata_, all the flowers are perfect, but each has a small calyx, and the whole are contained within a common involucre; and in the last order, _Monogamia_, the flowers are separated from each other. In Gynandria, the orders are determined by the number of the stamens. In Monoecia and Dioecia, the characters distinctive of the classes are employed for the orders. Polygamia has three orders, _Monoecia_, _Dioecia_, _Trioecia_; and the last class, Cryptogamia, is divided into four orders, consisting of the _Filices_ or _Ferns_, the _Musci_ or Mosses, the _Algae_, and the _Fungi_.