The Pteridophytes 



481 



Fig. 2g6. The shield fern {Aspidium marginale). 



rainy tropics. Most ferns are mesophytic and attain their 

 best development in partial shade and in rich humus soils. 



The ferns are readily distinguished by their divided and com- 

 pound leaves. The leaves arise near the apex of the stems and 

 uncoil as they develop. The youngest part of the leaf is 

 the apex. The venation is also characteristic, being forked or 

 dichotomous. 



The stems of most ferns are horizontal and branched, extend- 

 ing either at or below the surface of the soil. The cinnamon 

 fern of the United States has an erect stem, sometimes rising a 



