FERNS, FERN ALLIES, AND FOSSILS. 277 



{Collembold), which occurs in moist and shady places, 



seems to increase the number of young ferns, for more 



egg cells seem to be fertilised when this animal is 



present. After fertilisation the young fern plant 



develops from the ^%g cell. It has a special sucker-like 



organ which extracts everything that is useful from the 



parent prothallium, which very soon dies away. 



Thus the life story of the fern may be written as 



follows : Fern plant — spores — 



, „. /male spermatozoid. 



prothalhum<- f — ^ovum 



female egg-cell 



egg-cell — fern plant (compare Moss, p. 256). 



The chance of any one egg-cell becoming a fern 

 plant will be seen to be as follows : As one to the 

 number of archegonia on a prothallium, multiplied by 

 the number of spores in" a sporangia, multiplied by the 

 number of sporangia in a cluster, multiplied by 

 the number of clusters on a leaf, multiplied by the 

 number of spore-bearing leaves on a plant. It is 

 assumed that only one fern plant is produced by any 

 living fern, which is a quite justifiable assumption, as 

 the number of mature ferns in a country probably does 

 not perceptibly increase. The chance therefore of an 

 egg-cell becoming a fern is certainly not more than 

 I to 2,000,000, but the 1,999,999 wasted spores are 

 necessary, if one considers the multifarious dangers to 

 which these spores and prothallia are exposed. 



Clubmosses and Selaginellas — of which last there is one 

 British species — are allied to the Ferns. They have 

 rather thin, tough, and wiry stems, densely covered by 

 leaves, and are generally found either amongst alpine 

 plants, or on the peat in moorlands. Their sporangia 

 are found at the ends of the branches and are placed in 

 the axil of the leaves. These plants never occur in 



