12 ROMANCE OF LOW LIFE AMONGST PLANTS. 



From the foregoing- account it will be seen that the 

 germination of a fern differs materially from that of 

 ordinary plants, in that there is an intermediate 

 generation. This alternation has been called an 

 " alternation of generations," and something very 

 like it occurs in the animal kingdom. The spore on 

 germination produces a membranous prothallus. 

 Secondly, the prothallus produces male and female 

 organs, or fructification, by means of the germ-cells 

 and the spermatozoids. From the union of these 

 latter there proceeds a third generation, reverting to 

 the original type, whilst the intermediate is different. 

 Thus, then, we have a simple alternation of generations 

 between the normal frondose type and the protothal- 

 loid, which latter is of but short duration. Something 

 similar, but not so pronounced, takes place in the 

 germination of mosses, when a confervoid protonema 

 is developed, but of that anon. 



The climatic relations under which ferns in general 

 flourish, says Humboldt, "are manifested in the 

 numerical laws of their quotients of distribution. In 

 the plains within the tropical regions of large con- 

 tinents, this quotient is, according to Robert Brown, 

 and other more recent investigations on the subject, 

 '^Q of all phanerogamia, and in mountain districts of 

 large continents ^ to J. This ratio is quite different 

 on the small islands scattered over the ocean ; for 

 here the proportion borne by the number of ferns to 

 the sum total of all the phanerogamic plants increases 

 so considerably, that in the South Sea Islands the 

 quotient rises to \ ; while in the sporadic islands, St. 

 Helena and Ascension, the number of ferns is almost 



