Ill] THE BRACKEN FERN 45 



one another in a regular alternation. A produces B, 

 and then B again produces A : and so on. These 

 two generations differ widely in form and texture, 

 as also they differ in their water-relation. The 

 Fern-plant is structurally a land-growing plant, and 

 is capable of undergoing free exposure to ordinary 

 atmospheric conditions. Moreover dryness is essential 

 for the final end of its existence, viz. the shedding of 

 its spores. On the other hand the prothallus is 

 structurally ill fitted for exposure to dry air, while 

 fertilisation, which is the end of its existence, can 

 be achieved only in the presence of external fluid 

 water. 



Fertilisation may rightly be held to be the most 

 critical event in the whole life of the plant. It has 

 doubtless been a phenomenon which has recurred in 

 every completed cycle of life of Ferns throughout 

 their long course of evolution. Its absolute depen- 

 dence upon the presence of water is then a fact 

 that cannot be lightly put aside. The same is also 

 found in all such plants as Liverworts, INIosses, 

 Horsetails, and Club-i\Iosses, as well as in the Ferns. 

 Now the plants named constitute a Land-Vegetation 

 which, on grounds of comparison as well as on the 

 evidence of the geological record, is rightly held to 

 be more primitive, and to be lower in the scale of 

 evolution than the Seed-bearing plants. They occupy 

 a middle place between these and the Algae. The 



