68 



The Waterlilies. 



with the surrounding epidermis. The size is quite constant in the eight 

 species studied, namely about 0.0176 mm. in diameter ; those of N. tetra- 

 gona, however, are slightly oval and 0.02 to 0.025 mm. wide by 0.025 to 

 0.028 mm. in length. The average number of stomata per square centimeter 

 is, in round numbers, 340 for N. tetragona and odorata, 390 for N. tuberosa, 

 500 for N. zanzibariensis X , 530 for N. mexicana, 720 for N. ama- 

 zonum, 960 for N. alba candidissima, 1,000 for N. flava. Immediately 

 over the insertion of the petiole there is an area totally devoid of stomata, 

 Fig. 28, (t) ; it is of a rounded shape with a slender prolongation over 



Fio. 80. Leaf of N. rubra, vertical section ; o, air chamber ; Id, idioblast ; 

 p, palisade layer ; 8, stoma ; up, spongy parenchyma ; v, a small rein. 



each primary vein, and varies in size from 1.2 cm. to 3.8 cm. across. 

 Here the epidermal cells are slightly larger than elsewhere. At the edge 

 of this area the stomata are at first far apart and extremely large, Fig. 

 29, [c) ; in N. tetragona I found them 0.05 to 0.052 mm. long by 0.038 to 

 0.045 mm - wide, in iV. alba candidissima 0.045 mm - l n l by 0.038 mm. 

 wide, in N. odorata 0.043 to 0.045 mm - l n g by 0.038 to 0.043 mm - wide, in 

 N. zanzibariensis X 0.038 to 0.05 mm. long by 0.03 1 mm. wide, in N. ama- 

 zonum 0.031 to 0.038 mm. long by 0.028 to 0.033 mm. wide. From these 

 exceptionally large dimensions the stomata rapidly become smaller at 

 greater distances from the petiole, reaching the average size within the 

 length of 0.5 to 0.7 cm. A border of about 0.16 cm. in width around the 

 margin of the leaf is also devoid of stomata, but there is no increase in 

 size of those nearest the margin. 



