THE BRYOPHYTA: HEPATICAE, THE LIVER^VORTS 



399 



Marchantiales 



The INIarchantiales are Hepaticae in which the thallus is composed of 

 several distinct layers of tissue, of which the uppermost, the chlorophyll- 

 bearing layer, nearly always encloses air chambers, which have communica- 

 tion with the exterior through pores. The rhizoids are of two kinds, smooth 

 and tuberculate. The sex organs are generally united in receptacles often 

 borne on long-stalked antheridiophores and archegoniophores respectively. 

 The order includes relatively few British genera, though some of the species 

 are among the commonest Liverworts. 



We shall consider as our example of this order Marchantia polymorpha. 



Marchantia polymorpha 



Our second example of the Hepaticae, Marchantia polymorpha, is more 

 complicated than Pel/ia both in the structure of the thallus and also in the 

 sexual reproductive organs. The general morphology of the thallus of the 

 two plants is much the same (Fig. 381). It is flat, dichotomously branched, 



B 



Fig. 381. — Marchantia polymorpha. Habit photographs. A, Male thallus. 

 B, Female thallus about life size. 



possesses a central midrib and develops rhizoids on the lower surface in the 

 same way as in Pellia. There is, however, this considerable difference, that 

 there is no special apical cell, as in Pellia, but a group of meristematic cells 



at the apex. 



When we come to examine the internal anatomy of this thallus we find 

 that there is considerably greater differentiation of tissues. In Pellia the 



