THE COMMON FERNS 311 



305. The common ferns. The common ferns (order Filicales) 

 completely outclass all other orders of pteridophytes in num- 

 ber of species and mass of vegetation. The forms are exceed- 

 ingly various. The stems may be short and close to the ground, 

 or upright trunks, as in the tree ferns. But many types have 

 creeping stems, frequently wholly buried in the earth as under- 

 ground stems, or rootstocks, well illustrated by the common brake, 

 or bracken fern (Pteris aquilina). Some ferns have peculiar 

 methods of reproduction by buds that are formed on the leaves, 

 as in the bladder fern (Cystopteris bulbifera), or the walking 

 fern (Camptosorusj shown in Fig. 273. 



The fronds or leaves arise from the tip of the stem and form 

 clusters or crowns around the top of upright structures, but are 

 generally somewhat scattered along the creeping stems. Most 

 fronds are much cut or divided (compound) after regular and 

 various patterns (Fig. 271). They are developed very slowly 

 in some genera, remaining rolled up in the bud for several 

 months. However, when fully formed and in the proper season 

 they unroll comparatively quickly from the base in a very 

 characteristic manner until the apex finally appears above. 



The cell structure of the leaves, stems, and roots is very 

 much more complex than the cell structure in the bryophytes 

 and recalls at once the tissues of the seed plants (see Part I, 

 Chapters vn and xn). The plant body has a system of tissues, 

 called fibre-vascular bundles (Fig. 274), whose parts are much 

 modified for two important functions. One tissue is composed 

 of large cells (Fig. 274, t) empty of protoplasm arid with heavy 

 thickened walls marked with curious pits. These elements, 

 called tracheids, compose the woody part of the nbro-vascular 

 bundle termed the xylem, and their purpose is to conduct water 

 from the roots to parts of the plant above ground. But they 

 are also very important for the strength that they give to stems 

 and leaves. Another tissue is composed chiefly of cells, called 

 sieve tubes (Fig. 274, st), which contain much protoplasm and 

 food material and make up a softer region of the bundle termed 



