126 



THE BIOLOGY OF A PLANT. 



systems undergo great changes. The epidermis becomes very 

 thin, delicate, and transparent ; the fibro-vascular bundles break 

 up into an extremely fine and complex network forming the 



. 



FIG. 59.-Cross-section of part of a leaflet showing the microscopic structure. cp t 

 epidermis; st, stomata; f.s, intercellular spaces between the mesophyll-cells, 

 which are filled with (shaded) chlorophyll-bodies lying in the protoplasm. 



veins; the sclerotic tissues become transparent and are found 

 only along the veins. The cells of the fundamental parenchyma 

 alter their form, lose their starch, and become filled with bright- 

 green, rounded bodies, called the chromatoplwrcs or cliloroplnjll- 

 bodies, which are composed of a protoplasmic basis colored by a 

 pigment known as chlorophyll. The green fundamental paren- 

 chyma of the leaf is sometimes called the mesophyll. 



A cross-section of a leaflet (p. 100) is shown in Fig. 57. 

 The finer structure of the leaflet is shown in Figs. 58 and 59. 

 On the outside is the epidermis (V/>) ; within, the mesophyll and 

 midrib the latter composed of thickened epidermal and sclerotic 

 fundamental tissue, and a large fibro-vascular bundle. 



The mesophyll, or leaf-parenchyma, consists of irregular cells 



