STRUCTURE. 49 



space is filled with longitudinal air-canals separated by partitions of a 

 single cell-layer in thickness. Throughout the length of the stolon three 

 to five vascular strands pass along, each surrounded by endodermis with 

 the characteristic thickenings on the radial walls. G Wynne- Vaughan 

 (1896, a, b] called these strands "steles," and considered the stolons 

 polystelic. He states that the narrow isthmus connecting the tubers 

 of N. tnbcrosa with the parent axis are similarly polystelic. I am not 

 prepared to express an opinion upon the stelar nature of these strands, 

 especially since the present idea of a " stele " seems to be of doubtful 

 value, and, to say the least, needs more accurate definition. It will suffice 

 to describe the appearance of the vascular strands in transverse section, 

 and to mention their constituents. In N. flava a strand has at center a 

 large, regular and thickly cuticularized air-canal, with two or three 

 (usually three) projecting vascular lobes. Each lobe has one to three 

 (sometimes none) spiral xylem cells next to the cells bounding the air- 

 canal ; the rest of the area consists of phloem, with large sieve-tubes 

 and small companion cells. The vascular strands of N. rubra stolons 

 have the large air-canal with only one or two vascular lobes. 



In N. tuberosa the very short stolon resembles the rhizome in its 

 epidermis and superficial hairs, and the large, starch-laden cells of the 

 interior ; these latter cells include very large intercellular spaces. 

 There are about three vascular strands, bounded by endodermis. At 

 the middle of each, or near one side, is a large group of spiral elements, 

 corresponding in size and position to the air-canal in N. flava. A mod- 

 erate amount of phloem lies about this, either concentrated in four 

 phloem lobes, or half surrounding the xylem and bearing two or three 

 lesser lobes, or completely surrounding the xylem and scarcely lobed. 

 Two or three of these conditions are seen in a single section. 



The vascular system of nymphaeaceous stems has always been 

 described as consisting of scattered bundles (astelic), resembling mono- 

 cotyledonous structure. In most cases this is about all that can be said 

 (cf. Figs. 1 6, 1 8). Trecul (1845) studied the stem of Nuphar for a year 

 with considerable success, but was far from being satisfied with his results. 

 Nor do his researches shed much light upon the vascular structure of 

 Nymphaea. Although we can not yet unravel the complicated network 

 of tracheae and phloem strands, which seem to run in every imaginable 

 direction and position throughout the inner tissues of the stem, some 

 points are brought out by an examination of the stolons and young plants 

 of N. rubra, and the general and nodal structure of N.Jlava. This latter 



5 



