THE BRACKEN FERN 



107 



slowly into a horizontal underground stem. In time, new leaves grow 

 from the upper side of this stem and new roots from the lower side. 

 After the first of these new leaves and roots have appeared, the primary 

 leaf and primary root, which up to now have supplied food for the 

 plant, die and disappear. Now the asexual plant has reached the 

 condition at which we began to study it. 



133. Outline of the Life of the Fern. The fern, like the 

 moss, has a sexual generation which develops from a spore 

 and bears gametes, 



and an asexual gener- ,*'' *"* x 



ation which develops 

 from a zygote and * gametes 



bears spores. In 

 both moss and fern, 

 a spore can develop 

 only into a sexual 

 plant, and a zygote 

 can develop only into 

 an asexual plant. 

 The great difference 

 between the moss 

 and the fern is that 

 the sexual plant of 

 the moss is large and 

 long-lived, and its 

 asexual plant small and short-lived ; but in the fern the con- 

 ditions are just the opposite, the sexual plant being small 

 and relatively short-lived, and the asexual plant large and 

 long-lived. Thus what is commonly known as the moss 

 plant is the sexual generation; what is commonly known as 

 the fern plant is the asexual generation. 



There seems to be little doubt that the fern is descended 

 from some plant (perhaps a liverwort) which like the moss 

 had a long-lived sexual generation and a short-lived asexual 

 generation. In the evolution of the fern from this ancestor 



sexual 



generation asexual 



generation 



spores 



FIG. 59. Diagram illustrating the life 

 cycle of a fern. 



