76 



THE WATERLILIES. 



especially by lack of elaborated materials in the absence of mature foliage ; 

 cutting off the roots had a much less marked effect. I have noted also in 

 N'. rubra that young shoots acquire floating leaves much sooner if left 

 attached to the parent tuber than if early broken off and planted out. It 

 seems quite certain, therefore, that the occurrence of water-leaves in fall 

 and spring in Castalia is directly clue to the cutting off of the leaves by 

 frost, and the suspension of vitality in very cold water ; and, except in 

 the large and powerful tubers of N. flavo-virens, the young shoots of other 

 species must gather strength in themselves before they are able to pro- 

 duce floating leaves. In other words, a leaf-rudiment will, up to a certain 



Fio. 35. Transverse sections of petioles of water-leaves ; a. N. rubra, from tuber ; 

 b, N. (xlnrata, from rhizome in spring ; c, N. tuberosa, from tuber. 



age, develop into a submerged or a floating leaf according to nutritive 

 conditions. 



Such submerged leaves as are now under discussion assume a more 

 or less deltoid shape, varying to hastate (Lotos), cordate, or nearly orbicu- 

 lar (Fig. 34). Specific shapes, so far as known, are given in the taxo- 

 nomic section. The petioles are usually 2.5 to 7 or 8 cm. long, though 

 sometimes longer, and rather stout in proportion. Structurally they show 

 but slight differences from the petioles of floating leaves. The thickly 

 cutinized epidermal cells contain many bright-green chloroplasts in the 

 upper part of the petiole, where it is not covered by mud. The funda- 

 mental tissue, too, is rich in chlorophyll throughout. Its outer two to four 

 layers are more or less colloid in the angles, but only slightly so at most ; 

 here also chloroplasts are most plentiful. The arrangement of air-canals 

 differs from that of petioles of floating leaves, but in the larger water- 

 leaves this character approaches nearer and nearer to the typical arrange- 

 ment for the species. The tendency in petioles of water-leaves is toward 

 the condition with two main canals (Fig. 35) and a great many smaller ones 

 around and between these, with partitions but one cell thick. This 

 approaches the adult structure of N. flava and fctragona, and points also 

 to a probable primitive state in which, like Nuphar and the roots of 



