146 HETEROPHYLLY [CH. 



been recorded in R. sceleratus 1 ^ R. Lingua 2 and other species. 

 In R. sceleratus the present writer has observed that, in aerial 

 and in floating leaves, stomates occur on both surfaces, but in 

 the case of the floating leaf, the stomates were found to be 

 less numerous on the lower surface than in a leaf growing 

 in air. 



The heterophylly of the Nymphaeaceae has been discussed 

 in Chapter in 3 , so it is now only necessary to recall that 

 aerial leaves, floating leaves and submerged leaves may occur, 

 the latter belonging either to the t/A^-like type of Nymphaea 

 and Castalia, or the dissected type of Cabomba. 



Leaving the Ranales, it may be worth while to pass rapidly 

 in review the more pronounced cases of heterophylly met with 

 in the remaining families of Angiosperms. 



In Callitriche verna the submerged leaves are not very 

 different, superficially, from the floating leaves, but are 

 distinguished by their narrower and more elongated form 



(Fig- 94)- 



Hippuris vulgaris furnishes a particularly well-marked in- 

 stance of heterophylly. In May, when its flowering shoots rise 

 out of the water, there is the sharpest contrast between the 

 close whorls of rigid, short, aerial leaves (B-D in Fig. 95) and 

 the submerged whorls, with their long, flaccid leaves, visible 

 beneath the water surface (A in Fig. 95; see also Fig. 151, 

 p. 231). Goebel records that he once found Hippuris growing 

 entirely submerged at a depth of 3 metres, with leaves 7 cms. or 

 more long 4 . Towards July, when the plant is at its period of 

 maximum activity, the new shoots formed under water, even at 

 a depth of 50 cms., are reported to be of the aerial type and to 

 bear stomates 5 . This statement is of importance in connexion 

 with the problem of the significance of heterophylly, which will 



1 Ascherson, P. (1873), and Karsten, G. (i 



2 Roper, F. C. S. (1885). 



3 See pp. 27-29, and Figs. 12 and 14. 



4 Goebel, K. (1891-1893). 



5 Costantin, J. (1886). 



