246 THE PTERIDOPHYTA 



leaves becoming larger and larger, and more and more 

 complex in structure till they approximate to the size 

 and structure of the big compound leaves characteristic 

 of most kinds of adult ferns. The internal structure 

 of the body also increases in complexity, developing 

 the characteristic double vascular system. 



Our common ferns are mostly very much larger 

 than Liverworts or Mosses, and the Tree Ferns of 

 the tropics are often 20 or 30 feet high. This great 

 increase in stature is dependent on the development 

 of the vegetative characters described in the last 

 section, which provide a greatly improved equipment 

 for terrestrial life, and enable the fern to grow indefi- 

 nitely, constantly putting out new leaves and new 

 roots. The older parts of the plant body tend to 

 die off. 



The adult fern leaves (fronds) form spores similar to 

 the spores of Mosses and Liverworts. These are pro- 

 duced on the under sides of the fronds in little bag-like 

 structures (sporangia), which are formed in groups 

 (sori) usually covered with a membrane (Fig. 38, G). 

 The sporangia can just be seen with the naked eye. 

 When the spores are ripe the walls of the sporangia 

 split open and set the spores free to float in the air 

 (Fig. 38, H). On damp soil the spore germinates to 

 form the prothallus, which bears the sexual organs. 



Here it may be useful to summarise the life histories 

 of the Liverwort, the Moss and the Fern in the form 

 of a table (see opposite page). 



The plant bearing sexual organs is called the 

 gametophyte, the spore-bearing generation (free living 

 only in the case of the Ferns and other Pteri- 

 dophytes) the sporophyte. In the Moss the game- 

 tophyte is more highly adapted to terrestrial life than 



