336 PART III. THE CLASSIFICATION OF PLANTS. 



The Hepaticae are classified as follows : 



Order I. Marchantiaceas. Order II. Jungermanniaceae. Order III. Antho- 

 cerotaceae. 



Order I. Marchantiaceae. 



A. The GAMETOPHYTE. The spore gives rise on germination to a short un- 

 branched filamentous protonema which developes at its apex into a flattened 

 cellular expansion, from the margin of which the adult shoot (commonly 

 known as the plant) springs as a lateral branch. 



The Morphology of the Adult Shoot. The adult shoot is undifferentiated into 

 stem and leaf. Its symmetry is dorsiventral ; on the lower (ventral) surface it 

 bears numerous root-hairs, and also scales which are arranged in one or two 

 rows, or irregularly (Ricciocarpus natans, Clevea, Sauteria, Corsinia), but are 

 absent in Riccia crystallina ; on the upper (dorsal) surface the reproductive 

 organs are borne. In the Riccieae the shoot presents a dorsal furrow, and in 

 the Marchantieae a midrib. 



Growth is effected by an apical growing-point, situated in a depression, 

 possessing a transverse row of initial cells from which segments are cut off 

 dorsally and ventrally ; the initial cells also undergo longitudinal division, and 

 thus increase in number. 



The normal mode of branching is that which takes place in the plane of ex- 

 pansion ; it is dichotomous, and is effected in the manner described on p. 184. 



Less commonly, branches are formed as outgrowths from the ventral aspect 

 of the growing-point, as in certain Marchantieaa (e.g. Targionia, Fimbriaria, 

 Plagiochasma, Clevea, Preissia). This depends upon the fact that, in these 

 forms, the growth of the fertile normal branches is arrested by the formation 

 of reproductive organs : hence the further development of the shoot can only 

 be effected by ventral branching. Adventitious ventral branches are common 

 in the Riccieae. 



The sexual organs are in all cases developed on the dorsal surface, each 

 antheridium or archegonium arising from a single superficial cell. In the 

 simpler forms they are arranged in a continuous median row, developed in 

 acropetal succession (e.g. sexual organs of Eiccia, antheridia of Boschia, 

 Clevea hyalina, sometimes Sauteria alpina), or in a series of groups (e.g. Riccio- 

 carpus notans, Oxymitra, Corsinia, archegonia of Boscbia, antheridia of most 

 of the lower Marchantieae) ; in the higher forms they are borne on a special 

 structure termed a receptacle. 



The receptacle. The primitive form of tbe female receptacle is to be found 

 in Corsinia (var. involucrata). Here each group of archegonia is sunk in a pit 

 on the dorsal surface ; from the bottom of the pit there grows up among tbe 

 archegonia a protuberance, which produces a membranous umbrella-like cover- 

 ing (perichcetium) for them; this protuberance represents the receptacle. In 

 some of the lower Marchantieae (e.g. Plagiochasma, Clevea) the receptacle is 

 developed in much the same way, but the archegonia are borne upon it ; the 

 receptacle becomes raised on a short stalk and, by the unequal growth of its 

 surface, the archegonia (usually three to four in number) become displaced to 

 the under side. In these cases tbe receptacle is simply an excrescence of the 

 dorsal surface. 



In the higher Marchantieae the adult shoot is frequently differentiated into a 



