vii.] THE BRACKEN FERN. 61 



somewhat similar to that of Chara, but provided with many 

 more cilia. The antheridium bursts, and the antherozooids, 

 set free from their containing cells, are propelled through 

 the moisture on the under surface of the prothallus by their 

 cilia. 



The processes of the second kind acquire a more cylin- 

 drical form, and are called archegonia. Of the cells which 

 are situated in the axis of the cylinder, all disappear but 

 that which lies at the bottom of its cavity. This is the 

 embryo cell, and when the archegonium is fully formed, a 

 canal leads from its summit to this cell. The antherozooids 

 enter by this canal, and impregnate the embryo cell. 



The embryo cell now begins to divide, and becomes 

 converted into four cells ; of these, the two which lie at the 

 deepest part of the cavity of the archegonium subdivide and 

 ultimately form a plug-like, cellular, mass, which imbeds 

 itself firmly in the substance of the prothallus. Of the re- 

 maining two cells, which also undergo subdivision, one gives 

 rise to the rhizome of the young fern, while the other becomes 

 its first rootlet. It appears probable that the plug-like mass 

 absorbs nutritive matter from the prothallus, and supplies 

 the rhizome of the young fern, until it is able to provide for 

 itself. As the rhizome grows, and developes its fronds, it 

 rapidly attains a size vastly superior to that of the prothalius, 

 which at length ceases to have any functional importance, 

 and disappears. 



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 embryo cell by the antherozooid. 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 



