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LEAVES OF FILICALES 



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 



Fig. 220. Stem in early spring freed from all of its leaves save the young 

 ones, c, near the tip: r, roots; v, vascular bundles in base of petiole; x, region 

 from which the cortex has been removed to show the vascular bundles from 

 the leaves uniting to form a network. 



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 



Fig. 221. Cross-section of fern stem: v, concentric vascular bundle, the 

 large cells of the xylem being surrounded by the phloem. Each bundle is 

 surrounded by a compact layer of cells, the endodermis; st, stereome; cr, 

 cortical region. 



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 another feature about these leaves that 



