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. |
