- The gametophytes of Marchantiales differ from those of other Hepaticae in that they are internally differentiated into various tissues.
- The structure and development of the sex organs differ from that of all other Hepaticae but the Sphaerocarpales.
- The sporophytes are also distinct from other Hepaticae except the Sphaerocarpales in that the jacket layer surrounding the sporogenous tissue is but one cell in thickness.
- The order includes about 25 genera and 400 species.
- The gametophytes of Marchantiales are ribbon-shaped, dichotomously branched, and dorsiventrally differentiated.
- From the standpoint of complexity of internal structure, they arc the most advanced of the
- The dorsal part of the thallus is internally differentiated into air chambers, opening externally by a pore, and the ventral part is internally differentiated into a storage tissue comprised of two or three types of cells.
- In some cases there is a rudimentary conducting tissues of elongate cells in the sagittal axis of the storage tissue.
- Gametophytes of most Marchantiales differ from those of other Hepaticae in the, presence of two kinds of rhizoids and in the presence of scales on the ventral surface.
- In addition to these vegetative features all members of the order have antheridia developing in the same manner as will be described for Riccia. The archegonia also differ from those found in most other genera of Hepaticae in that the neck typically consists of six tiers of cells.
- Sporophytes of Marchantiales differ from those of other Hepaticae (except Sphaerocarpales) in that the jacket layer never becomes more than one cell in thickness. From the standpoint of complexity of sporophytes, the order includes the family (Ricciaceae) with the simplest known sporophytes.
- The simplicity of this sporophyte is generally ascribed to its primitiveness, but there are those l who consider it a reduced rather than a primitive type.
If one accepts,. the primitiveness of the Ricciaceae, two general tendencies are evident in the Marchantiales. One of these is in the sporophyte.
- From the simple condition where the entire sporophyte is a capsule, there has been a partial sterilization of the sporogenous tissue to form a foot and a seta.
- In addition, advanced members of the order have a further differentiation of the sporogenous tissue into cells which produce spores and sterile cells (elaters) which assist in spore dispersal
- The other general evolutionary tendency is in the manner in which the archegonia are borne. In the most primitive family, the Ricciaceae, the archegonia are borne in a longitudinal dorsal strip of indefinite extent.
- The family above this, the Corsiniaceae, produces archegonia in definitely circumscribed areas (receptaCles), but a succession of receptacles may be formed by a branch.
- Intermediate families (Targioniaceae and Monocleaceae) have but one receptacle on a branch, and this lies just posterior to the growing apex. The most advanced family, the Marchantiaceae, also has a restriction of the female receptacle to a branch apex, but it
only produces female receptacles on special erect branches.
- The nature of the receptacle, the manner in which it is borne, and the structure of the
sporophyte are the bases for a segregation of the order into five families.