PRINCIPAL GROUPS OF PLANTS. 91 



greenish or yellowish in color, variously sculptured, and vary 

 from 0.025 mm. to 0.158 mm. in diameter. 



Fern Groups. There are a number of distinct groups of 



FIG. 54. Several Osmundas. i, the royal fern (0. regalis) showing fertile tip of 

 branch and sterile bipinnate leaflets below; 2, Clayton's fern (O. Claytoniana) showing 

 three pairs of fertile leaflets in the middle and a number of sterile leaflets above and below; 

 3, cinnamon fern (O. cinnamomea) showing a fertile leaf (sporophyll) to the left and a sterile 

 leaf (foliage leaf) to the right, 



ferns which vary considerably in appearance. ( I ) In the Tropics 

 as well as in greenhouses TREE FERNS, characterized by an over- 

 ground stem, occur. The leaves arise at the summit of the stem 

 or trunk and form a crown. 



