82 The Waterlilies. 



by a crown of ovate cells with dense granular contents. Intermediate 

 forms were seen. 



In N. odorata the sepal is quite thick at the base, but very thin above. 

 In the basal region there are about 1 2 tiers of cells between the outer 

 epidermis and the air-canals, of which the first 4 or 5 are slightly 

 collenchymatous. They are all rounded in outline and become gradually 

 larger from without inward. Among them are very many bipolar idioblasts 

 lying lengthwise of the sepal. The air-canals are very numerous and 

 irregular in shape and size. They are separated by partitions only one 

 cell thick, and occupy a zone about as thick as the denser parenchyma 

 just described. Great numbers of idioblasts of irregularly stellate forms 

 occur here. Between this zone and the inner epidermis there is a zone of 

 about 6 layers of parenchyma. A single irregular row of vascular bundles 

 traverses the partitions between the air-canals. They have well-developed 

 protoxylem and, in all the larger ones, an air-canal. The same structure 

 is continued in the apex of the sepal, but with fewer tiers of cells and 

 much smaller air-passages. Nearly all of the parenchyma cells contain 

 chloroplasts, especially the two or three outer layers, where the cells are 

 crowded together and polygonal in outline. 



A section near the middle of the sepal of N. tuberosa shows a still 

 more leaf-like arrangement. Within the outer epidermis are two to four 

 layers of round cells with large intercellular spaces and containing many 

 chloroplasts. Then comes a zone 3 to 5 times wider than the first, made 

 up of branching lines of cells and large intercellular spaces. Through 

 this zone the vascular bundles travel, accompanied by one or two layers 

 of close-fitting parenchyma. Phloem and xylem are well represented, but 

 no air-canal was seen. The orientation of the bundles is normal in all 

 parts of the flower. Stellate idioblasts are plentiful in the air-chambers. 

 The inner epidermis is backed by a single continuous layer of rounded 

 cells. Near the apex of the sepal the air-chambers are quite small, the 

 intervening partitions are multicellular, and idioblasts much fewer. The 

 total thickness of the sepal, however, may be greater than at the middle. 



Most of the general features of the petals have been already men- 

 tioned, and specific details are sufficiently covered in the chapter on taxon- 

 omy. The outer four petals are usually of about the size and shape of the 

 sepals. They are thicker at base than elsewhere, and are traversed by a 

 number of longitudinal veins which divide up into a fine network in the 

 upper half or two-thirds of the petal. There may be some longitudinal 

 air-canals in the basal parts. No distinction of blade and claw occurs, but 



