HISTOLOGY OF THE LEAF. 



125 



portions at its base from the outer world. Protected by these hairs, the 

 end of the stem forces its way through the toughest clay without injury to 

 the delicate bud buried in its apex. (Hofmeister.) 



FIG. 58. Cross-section, still more enlarged, passing throngh the midrib of a leaflet. 

 In the centre the circular nbro-vascular bundle, supported, especially above and 

 below, by thickened prosenchyma ( p>. On either side the parenchymatous, mes- 

 ophyll cells (shaded) and the intercellular spaces (i.*) opening by stomata (st); 

 epidermis <ep). 



THE AERIAL PART OF THE BRAKE. THE FROND OR LEAF. 



The external form of the leaf has been described on p. 109, 

 and it now remains to consider its internal structure. The 

 lamina is to be regarded as a flattened and altered portion of the 

 stipe, made thin and delicate in order to present a large surface 

 to the light and the air. The stipe, in turn, is a prolongation 

 of the rhizome, so that the whole plant body is a continuous 

 mass, throughout which extend the three systems of tissue vir- 

 tually unchanged. The transverse and longitudinal sections of 

 the stipe show only minor points of difference from correspond- 

 ing sections of the rhizome. In the leaf, however, all three 



