PLANT CLASSIFICATION 



161 



very closely. The pear genus consists of the pear species, the apple 

 species, and the crab apple species. Species, again, may be still 

 further subdivided into varieties, in which the plants are more closely 

 related (e.g. Baldwin and Greening varieties among apples) . And 

 finally a species (or variety) is made up of individual plants, that 

 resemble each other in all essential respects. 



Sub-kingdom II, Spore-producing Plants (Optional) 



A. Ferns 



176. The fern plant. 

 We turn now from a dis- 

 cussion of seed-bearing 

 plants to a consideration 

 of those plants which 

 never produce flowers or 

 seeds. As a representa- 

 tive of the highest group 

 of plants without seeds, 

 we will study the ferns. 

 The majority of ferns 

 grow in damp, shady 

 places, and among the 

 common kinds we may 

 name the brake, the 

 maiden-hair, and the rock 

 fern. In any one of these 

 ferns the parts above 

 ground which are true 

 leaves are known as 

 fronds. The main axis 

 of each frond runs 

 throughout the leaf, and 

 to each side are attached 

 the leaflets, which may or 



FIG. 82. Fern plant (Aspidium), showing 

 roots, rhizome, and frond : A, 'section of 

 fruit dot (sorus) , showing spore cases, some 

 of which are ejecting their spores ; B, por- 

 tion of a leaflet, showing unripe fruit dots ; 

 C, portion of a leaflet, showing ripe fruit 

 dots. (Strasburger.) 



