THE PTERIDOPHYTES 



279 



quite closely. In the lower epidermis, rarely in the upper, are 

 the stomata. By means of a transverse section of a leaflet 

 (Fig. 231) the other leaf tissues are seen to be (1) the veins, 

 which appear in cross section as bundles of very small, heavy- 

 walled cells ; and (2) the chlorophyll-bearing cells. The upper- 

 most chlorophyll-bearing cells are long and stand close together 

 Qpalimde tissue'), with their ends at right angles to the sur- 

 face. The other chlorophyll-bearing cells are loosely arranged, 

 (the spongy tissue) and between them are many air spaces. 



Fig. 231. A cross section of part of a fern leaf, showing the indusium (i) 

 and sporangia (s) on the underside 



After Engler and Prantl 



259. Importance of introduction of fibrovascular tissue. It 



is evident that a fern leaf exposes much chlorophyll to the 

 light, — much more than did any plant among the bryophytes. 

 The strong supporting and conducting tissues of the leaf 

 uphold the chlorophyll tissues in such a position that they 

 may receive light; at the same time, tlxrough the fibrovascular 

 bundles of the leaf, soil water may be transported up to the 

 chlorophyll tissue. The root system anchors the plant in the 

 earth and absorbs the water needed in food manufacture. 



