186 PRACTICAL BOTANY. 



lar neck leading to the oosphere. By the fertilization of 

 the oosphere a new plant is produced. This plant in turn 

 produces spores. The life history, as just detailed, is 

 believed to be practically the same in all Pteridophytes,^ 

 though it has not been fully observed in all the species. 

 There are three classes. 



CLASS I. THE TRUE FERNS (Filicince). 

 CLASS II. THE HORSETAILS (Equisetince). 

 CLASS III. THE CLUB MOSSES (Lycopodince). 



CLASS I. The True Ferns. Two groups of plants 

 included here are heterosporous, i.e., they have two kinds 

 of spores, female macrospores and male microspores. These 

 are the Salvinacece and Marsiliacece. All others are homo- 

 sporous or isosporous, i.e., they bear only one kind of spore. 



The Ferns, in the narrower sense of the term, are dis- 

 tinguished at once from the other members of this branch 

 by the luxuriance of their leaves. The stem, or rhizome, 

 is underground and develops by the formation of a new 

 bud at one end each year, while it dies away at the older 

 end so as about to keep pace with the new growth. In 

 the spring the young fronds may be seen unrolling them- 

 selves, for they are coiled up, or circulate, in the bud. 

 Ferns vary in height from an inch up to many feet, the 

 larger ones, in tropical climates, often raising the rhizome 

 out of the ground to form the trunk of a tree with a tuft 

 of fronds at its top. 



On account of the delicate beauty of their fronds, Ferns 

 offer a very attractive field for study. It is estimated 

 that there are from 3000 to 4000 species in all the world. 

 They flourish most abundantly in the torrid zone, decreas- 

 ing in number and size as one passes from the equator 

 towards the poles. Only 26 species are known to grow 

 within the limits of the Arctic zone, and only 67 species 

 are recorded for the whole of Europe. For North America. 



