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MORPHOLOGY 



evolution. It is this gradual disappearance that must be noted in 

 connection with subsequent groups. 



(3) ANTHOCEROTALES 



General character. This is a small group comprising four genera: 

 Anthoceros and Notothylas of the temperate regions ; Dendroceros, an 

 epiphytic tropical genus; and Megaceros, a genus recently described from 

 Java. Although few in numbers, the group is of great morphological in- 

 terest on account of the claims made for it that it possibly represents the 

 ancestral forms of pteridophytes. Its possible relation to the mosses 

 also further emphasizes its important genetic position. It differs so 

 much from the other liverworts as to have suggested its separation from 

 them as a third great group of bryophytes, coordinate with liverworts 

 and mosses. In Marchantiales and Jungermanniales there is extensive 

 differentiation of the body of the gametophyte, either in structure or in 

 form; but in Anthocerotales there is a simple gametophyte, while the 

 sporophyte is the most complex among liverworts. 



Gametophyte. The body of the gametophyte is a simple thallus (figs. 

 239, 240), almost as simple as that of Aneura, and much simpler than 

 that of Marchantia. The margin is often wavy, lobed, or crisped ; and 



in Dendroceros the lobing in 

 some cases suggests rudi- 

 mentary leaves. The thallus 

 matures by means of an 

 apical cell with four cutting 

 faces, the preceding stages 

 appearing as usual. There 

 are two marked peculiarities 

 of the gametophyte body in 

 most of the genera : (i) the 

 usually single large chloro- 

 plast, generally in contact 

 with or even more or less 

 investing the nucleus; and 

 (2) the mucilage cavities, 

 which open by clefts on the 

 /entral surface. In these cavities endophytic Nostoc colonies occur. 



The sex organs are developed on the dorsal side of the thallus, but in 

 ;ertain features they differ strikingly from those of other bryophytes. 



FIG. 237. Anthoceros: an antheridial chamber 

 containing three antheridia. 



