These may be conveniently arranged in four classes, as follows:—
|I. Cryptogamia; acotyledonous, asexual, or flowerless:||Stem and leaves undistinguishable:||1. Thallogenae.|
|Stem and leaves distinguishable:||2. Acrogenae.|
|II. Phanerogamia; sexual, or flowering:||Cotyledons solitary or alternate; wood of stem youngest in the centre:||3. Endogenae.|
|Cotyledons opposite or verticillate; wood of stem youngest at the circumference:||4. Exogenae.|
The Elements of Materia Medica and Therapeutics, Vol. II, 3th American ed., was written by Jonathan Pereira in 1854.