312 THE PTERIDOPHYTES 



bast or phloem. The bast regions are known to be paths for the 

 distribution of food material in the plant. The structure of the 

 fern frond is essentially similar to that of the leaf of a seed 

 plant. There are stomata on the lower surface and chlorophyll- 

 bearing tissues underneath the outer cell membrane, or epidermis. 

 The fibro-vascular bundles run out into the green expanded por- 

 tions of the leaves as forking veins, which do not, however, 



FIG. 274. Fibro-vascular bundle from the underground stem, or rootstock, of 

 the common brake (Pteris aquilina) 



g, ground tissue, or parenchyma ; bs, bundle sheath ; ps, hast, or phloem, sheath 

 surrounding the sieve tubes (st) and bast fibers which compose the bast, or 

 phloem ; t, large, thick-walled cells called tracheids, which with smaller cells 

 in the center make up the wood, or xylem 



generally unite to form the close network so characteristic of 

 dicotyledonous seed plants. 



As stated above, the fibro-vascular bundles greatly strengthen 

 the tissues of the leaves and stems, for they form a sort of 

 skeleton in the plant. They are frequently assisted in their 

 strengthening functions, especially in the stem, by regions of 

 rigid tissue, which may be variously situated, sometimes under 



