THE BIOLOGY OF A PLANT. 



finer structure of the leaflet is shown in Figs. 46 and 47. On the 

 outside is the epidermis (ep) ; within, the mesophyll and mid- 

 rib the latter composed of thickened epidermal and sclerotic 

 fundamental tissue, and a large fibre-vascular bundle. 



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

 which are loosely arranged on the lower side leaving very large 

 intercellular spaces, but are closely packed, and leave few or no 

 intercellular spaces, on the upper (sunny) side. The cells have 

 very thin walls, contain protoplasm and a large central space 

 (vacuole) filled with sap, and numerous chlorophyll-bodies im- 

 bedded in the protoplasm. These are especially numerous in 

 the upper part of the leaf, as might be expected from their 

 functions in connection with the action of light (see page 106). 



The epidermis, or skin of the leaf, consists of translucent, 

 greatly flattened cells having peculiar wavy outlines and rela- 

 tively thick walls (Figs. 36, 46-49). 

 Upon the veins they become elongated, 

 and their walls are considerably thick- 

 ened, especially upon the midrib (Figs. 

 46, 48). They generally contain large, 

 distinct nuclei, and often considerable 

 protoplasm. The wavy epidermal 

 cells, particularly in young plants, 

 contain some chlorophyll and starch, 

 though in this respect the fern is some- 

 what exceptional. 



In the rhizome the epidermis forms 

 a continuous layer over the whole sur- 

 face. In the leaf, however, this is not 

 the case, the epidermis on the lower 

 side being perforated by holes leading 

 into the interior and known as mouths 

 or stomata (singular, stoma) (Fig. 48). 

 These holes do not pass into the cells, 

 but are gaps or breaks between certain cells of the epidermis, and 

 open directly into the intercellular spaces of which they are, in 

 fact, the ends. That portion of the intercellular labyrinth which 

 directly underlies the stoma is sometimes called the respiratory 

 cavity. Each stoma is bounded, as in most plants, by two curv- 

 ing guard-cells, w^hich are generally nucleated, and, unlike epi- 



FIG. 49. (After Sachs.) Epidermal 

 cells of Pterisflabellata, showing 

 the development of stomata. A, 

 very young epidermal cells; B, 

 nearly mature: i.c, initial cell; 

 m.e, mother-cell; s.c, subsidiary 

 cell; y.c, guard-cell; st, stoma. 



