THE BIOLOGY OF A PLANT. 



AERIAL PART OF THE BRAKE. LEAF. 



The external form of the leaf has been described on p. 68, 

 and it now remains to consider its internal structure. The 

 lamina is to be regarded as an expanded 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 from 

 the rhizome, so that the whole plant is a continuous mass, 

 throughout which extend the three systems of tissue virtually 



FIG. 47. Cross-section of part of a leaflet showing the microscopic structure. e/>, epidermis; 

 st, stomata; i.s, intercellular spaces between the mesophyll-cells, which are filled with 

 (shaded) chlorophyll-bodies lying in the protoplasm. 



unchanged. The transverse and longitudinal sections of the 

 stipe show only minor points of difference from corresponding 

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

 undergo great changes. The epidermis becomes very thin, 

 delicate, and transparent ; the fibre-vascular bundles break up 

 into an extremely fine and complex network forming the veins / 

 the sclerotic tissues become transparent and are found only 



