XV.] THE BRACKEN FERN. 451 



the embryo is attached to the prothallus and derives food- 

 supphes from it. 



As the rhizome grows, and developes its fronds, it rapidly 

 attains a size vastly superior to that of the prothallus, which 

 at length ceases to have any functional importance, and dis- 

 appears. 



Thus Pteris presents a remarkable case of the alternation 

 of generations. The large and complicated organism com- 

 monly known as the ' Fern ' is the product of the impreg- 

 nation of the ovum by the spermatozoid. This 'Fern,' 

 when it attains its adult condition, developes sporangia; 

 and the inner cells of these sporangia give rise, by a perfectly 

 asexual fissive process, to the spores. The spores when set 

 free germinate; the product of that germination is the in- 

 conspicuous and simply cellular prothallus ; an independent 

 organism, which nourishes itself and grows, and on which, 

 eventually, the essential organs of the sexual process — the 

 archegonia and antheridia — are developed. 



Each impregnated ovum produces only a single 'fern,' 

 but each 'fern' may give rise to innumerable prothalli, 

 seeing that every one of the numerous spores developed in 

 the immense multitude of sporangia to which the frond gives 

 rise, may germinate. 



LABORATORY AVORK. 



A. The Fern-plant ; asexual generation. 



a. External characters. 



a. The brown underground stem or rhizome, with 

 a lighter band (the lateral li?ie) running along 

 each side of it : its nodes and iiiternodes. 



29 — 2 



