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A TEXTBOOK OF THEORETICAL BOTANY 



filament, from which a fresh Marchantia thallus is produced. Since the sex 

 organs are developed on different plants it follows that the spores must be 

 of two kinds. As we shall see later in our consideration of the Laws of 

 Inheritance it is probable that male and female producing spores arise in 

 equal numbers, but female plants are commoner than males. 



Alternation of Generations 



The life history of Marchantia, it will be seen, is similar to that of Pellia. 

 Apart from the purely asexual pioduction of gemmae there is a definite 

 alternation between a gametophyte, the thallus on which the gametes are 

 borne, and a sporophyte in which the spores are produced. In so far as the 

 sporophyte develops within the archegonium and derives its nourishment 

 therefrom there is little difl^erence in the two examples. In fact a relatively 

 small and parasitic sporophyte is characteristic of the whole of the Bryophyta, 

 and it is not until we pass to the Pteridophyta that we shall find any change 

 in its importance. 



The life-cycle of Marchantia may be illustrated graphically by the 

 following diagram (Fig. 391) : — 



. Oospore 



Sporogonium 



Spores 



Marchantia 

 Female Thallus 



Marchantia 

 Male Thallus 



Antheridium 



Antheridiophore 



Fig. 391. — Life-cycle oi Marchantia polymorpha. 



Anthocerotales 



The Anthocerotales are Hepaticae in which the thallus is simple and 

 undifferentiated in structure. The sporogonia are remarkable for their 

 continued growth from a basal meristem. They are long and narrow with 



