Tropophytes.These plants are characterized by being xerophytic during the unfavourable season.
Hygrophytes.Living, as these plants do, under medium conditions as regards soil, moisture and climate, they exhibit no characters which are markedly xerophytic or hydrophytic. Hence, such plants are frequently termed mesophytes.
The plants grow as a rule in marshy places, though some species of Marsilia are xerophytic. The creeping stem produces roots from the ventral surface and leaves from the dorsal surface; the leaves when young are circinately coiled.
The latter shows marked xerophytic adaptations; the single vascular bundle was surrounded by a sheath of short tracheides, and the stomata were sheltered in two deep furrows of the lower surface.
All such plants agree in reducing transpiration to zero during the unfavourable season, although few or no xerophytic characters may be demonstrable during the period favorable to growth.
