The lowest Hepaticae have an extremely simple vegetative structure, little more advanced than that found in some of the higher Green Algae and very much simpler than in the large Red and Brown Seaweeds.
For the carrying on of their functions they all need to be supplied with carbohydrates or other carbon compounds which they obtain ordinarily from humus and plant residues in the soil, or possibly in some instances from carbohydrates manufactured by minute green algae with which they live in close union.
On the other hand, it has been held by Bernhard Frank and other observers that atmo spheric nitrogen is fixed by the agency of Green Algae in the soil.
Similar modes of growth occur among the Siphoneous Green Algae and also among the Red Seaweeds.
The vesicles of Fucaceae and Laminariaceae prevent the sinking of the bulkier forms. But why certain Fucaceae favour certain zones in the littoral region, why certain epiphytes are confined to certain hosts, why Red and Brown Algae are not better represented in fresh water or Green Algae in salt, - these are problems to which it is difficult to find a ready answer.
Again, many Green Algae - some unicellular, like Sphaerella and Chlamydomonas; some colonial forms, like Volvox and Hormotila: some even filamentous forms, like Ulothrix and Stigeoclonium- are known to pass into a condition resembling that of a Palmella, and might escape identification on this account.
Among the Green Algae the differentiation of cells is comparatively slight.