Structure. 



63 



side of the canals equidistant from each other and from the former two. 

 According to the size of the petiole there may be an additional, still 

 smaller bundle in the middle of each space between the six just located ; 



Fio. 27. Types of vascular bundle; (a) simple bundle; 



(b) small bundle with air-canal from petiole of N. luberota; 



(c) double bundle from peduncle of N. zanzibariensis ; (d) 

 double bundle with two canals, petiole of N. dentata. a, air- 

 canal ; 8, sieve tube ; sp, spiral trachasa ; t, tannin or latex cell. 



and still another series of twelve very slender 

 ones may alternate with all of the preceding 

 ones. Where four main air-canals are present, 

 there is always a large bundle in the middle 

 of the petiole where the four partition walls 

 meet ; the peripheral bundles are arranged on 

 the same principle as before, though usually 

 there is but one large lateral bundle on each 

 side ; thus the number of peripheral bundles 

 will be a multiple of four instead of six. 



In structure the vascular bundles of 

 the petiole (and peduncle) of Nymphaea 

 show that extreme reduction of xylem which usually characterizes water 

 plants. In the absence of any distinguishable endodermis, the bundles 

 are sheathed by ordinary, large, fundamental cells ; occasional inter- 

 cellular spaces occur between these and the bundle-cells, but this is 

 not common ; the outer side of the sheathing-cells, however, joins with the 



