322 



LEAVES OF FILICALES 



trunks of trees, as well as upon the earth. The leaves are the 

 most striking feature of the Filicales. They are usually large and 

 divided and are characterized by being coiled when young (Fig. 

 219). This is due to the stronger growth of the outer cells which 

 causes an inward rolling of the leaf. The growth of the leaf is 

 very slow, often requiring three years for its formation in tem- 



FlG. 219. Christmas fern, Polystichum, with prostrate stem bearing at 

 the tip young coiled leaves covered with chaffy scales and further back large 

 leaves of the previous season. The older portions of the stem are covered 

 with the petioles of the dead leaves. H. O. Hanson. 



perate regions. During the season preceding its expansion, the 

 petiole and blade are completely formed and appear in crosier- 

 like coils more or less covered with chaffy scales (Fig. 220, c). 

 This development enables the leaves to expand with surprising 

 rapidity in the spring when the more rapid enlargement of the 

 cells on the inner side of the leaf cause it to uncoil. There is 



