292 NATURE OF THE FILICALES 



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 



FIG. 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 dead leaves. H. O. Hanson. 



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 temperate 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 



