60 STRUCTURAL BOTANY 



st). The annulus not only straightens but bends back 

 on itself in the opposite direction. The contraction of 

 the ring and bursting of the sporangium takes place with 

 so much violence as to forcibly eject the spores, which 

 are scattered abroad and may be carried to a great 

 distance by the wind. 



On the island of Krakatoa (Malay Archipelago), the 

 vegetation of which was completely destroyed by the 

 volcanic eruption of 1883, and which lies about eleven 

 miles from the nearest land, Ferns were among the very 

 first plants to reappear after the catastrophe. 



It is a very general rule that the dehiscence of 

 sporangia is so contrived as to take place in dry 

 weather ; the advantage of this to the plant is obvious. 

 When the air is dry the spores form a powdery 

 dust, which is easily scattered by the wind, whereas 

 in wet weather they hang together in damp clusters, 

 and could never be properly disseminated. 



We have now traced the history of the reproductive 

 process in the asexual generation. The most' important 

 points in which the Male Fern differs from Selaginella 

 are the totally different arrangement of the sporangia, 

 the origin of each sporangium from a single cell, and the 

 fact that sporangia and spores are all of one kind. 



As regards the two latter points, however, all Ferns 

 do not agree with the Male Fern, for in some members 

 of the class the sporangia have a multicellular origin, 

 while others are heterosporous. It now remains for us 

 to follow the germination of the spores, to see how the 

 prothalli are produced from them, to learn how fertilisa- 

 tion is effected, and finally to study the origin of the 

 embryo, which develops once more into the asexual Fern- 

 plant, and thus completes the cycle of life. 



