STRUCTURE. 



67 



midrib near the base forms a ridge a centimeter high and 0.3 to 0.5 cm. 

 wide, with vertical, parallel sides and rounded summit. The primary veins 

 are but little lower ; the ultimate branches also, especially where they run 

 out into the serrations, stand out like little cords. Thus by a system of 

 underlying rafters the lamina is rendered firm and stiff. In the large 

 leaves of Euryale and the gigantic foliage of Victoria this method of 

 strengthening is carried to a remarkable degree of development. 



In describing leaf-structure we may speak of (i) the upper epider- 

 mis, (2) the palisade layer, (3) spongy parenchyma, (4) idioblasts, (5) vein- 

 structure, (6) lower epidermis, and (7) external hairs. 



ftzj 



Fio. 29. Epidermis of leaf, N. tctragnna: (a), lower epidermis with bases of 

 hairs ; (t>), upper epidermis with average stomata ; (c), upper epidermis with very 

 large stoma from margin of astomatic area. 



The upper epidermis is perfectly smooth and even and somewhat 

 cuticularized on its outer surface. The cells are square or polygonal and 

 from 0.0127 to 0.0178 mm. across (in eight species and varieties). Their 

 outlines are quite wavy in N. flava, and mexicana, less so in N. tetragona 

 and alba candidissima, and nearly straight in N. aniasonum, odorata, tube- 

 rosa and zanzibariensis X . The cells vary in depth, but are about o.oi mm. 

 deep in N. rnbra and odorata, o.oi to 0.013 mm. in N. tnbcrosa, 0.013 in 

 N. amazomim, o.oi to 0.0178 mm. in A r . tetragona, and 0.0178 to 0.02 mm. 

 in N, zanzibariensis x . Stomata are present only on the upper surfaces 

 of the floating leaves. They are usually small and round, Fig. 29, (b), 

 and separated from one another by two to three or four epidermal cells ; 

 their openings are comparatively large. The guard-cells stand on a level 



