338 PART III. THE CLASSIFICATION OF PLANTS. 



rise to antheridia in acropetal succession, and then ceases to growj'the stalk Las 

 two ventral furrows, showing that it consists of two coherent branches. 



In all cases the antheridia, and in many the archegonia, become overgrown 

 by the surrounding tissue, so that they lie in depressions formed in essentially 

 the same manner as the air-chambers described below ; a narrow canal leads 

 down to each antheridium, but the necks of the archegonia are long enough to 

 reach to the surface. The growth of this tissue is especially active around the 

 archegonia, so that a membranous investment is formed enclosing either a 

 single archegonium, when the archegonia are developed singly, or a group of 

 archegonia, when they are developed several together, which is termed the 

 involucre. This consists essentially of two parts ; a basal portion, consisting 

 of an extension of the air-chamber-layer of the shoot, and of a more delicate 

 marginal portion (pericbaetium), formed by the outgrowth of the superficial 

 cells of the basal portion, variously lobed and laciniate. In some cases (e.g. 

 Corsinia) the development of the perichaatium only takes place after fertilisa- 

 tion. 



In the Marchantiero, the receptacles are surrounded at their first formation 

 by invohicral scales of various form, leafy or hair-like, which are excrescences of 

 the adjacent tissue. When the receptacle subsequently becomes raised on a 

 stalk, the involucral scales generally remain as a sheath round the base of the 

 stalk ; but some may be carried up with the stalk in its growth, as is especially 

 the case in Lunularia. 



In Marchantia, Preissia, and Fimbriaria, the venter of each archegonium 

 becomes surrounded by a sac-like membrane, developed from the stalk-cell of 

 the archegonium, which is termed the perigynium (Fig. 235). The develop- 

 ment of the perigynium begins when the archegonium is nearly mature. 



The distribution of the sexual organs is various : the plants maybe monoecious 

 (some species of Riccia, also Corsinia, Plagiochasma, Sauteria, Peltolepis, 

 Grimaldia, Eeboulia, most species of Fimbriaria, Duvallia, Targionia, Cyatho- 

 dium), or dioecious (some species of Eiccia, Ricciocarpus no-tans t Oxymitra, 

 Boschia, Clevea, Fegatella, Lunularia, Preissia generally, Marchantia, Du- 

 mortiera). In the monoecious species of Kiccia (except R. fluitans, where 

 they alternate regularly in a single row), the antheridia and archegonia are 

 intermingled ; but in all other monoecious forms they are developed either in 

 separate sori, or on distinct branches (diclinous) ; in Preissia both antheridia 

 and archegonia have been found to occur exceptionally on the same (andro- 

 gynous) receptacle (monoclinous). The antheridia are borne, in Sauteria, 

 Targionia, and Cyathodium, on the ventral branches exclusively ; it is probable 

 that this is also the case in the apparently dioecious forms of Fimbriaria. 



The Structure of the Adult Shoot. The dorsal portion of the shoot consists, 

 in all the Marchantiaceae, of parenchymatous tissue, made up of cells containing 

 ehloroplastids, which includes a number of air-chambers, giving it an areolated 

 appearance, whence it is termed the air-chamber-layer. The chambers are 

 formed by the unequal growth of the cells near the growing-point, in conse- 

 quence of which the surface presents alternating elevations and depressions. 

 In most species of Eiccia the air-chambers are narrow canals, elongated per- 

 pendicularly to the surface ; in R. crystallina they are much widened, and open 

 to the surface by the whole extent; in R.fluitans (Ricciella canaliculata) the 



