xi] SUBMERGED LEAVES 147 



be discussed later in the present chapter. When winter comes 

 on, the thin, submerged, stomateless type of leaf is again 

 produced. Fig. 96, p. 148, represents a rather curious case, 

 in which a shoot had reverted to submerged leaves (a) after 

 bearing aerial leaves (c). It had apparently been beaten down 

 into the water by heavy rains, and this involuntary return to 

 submerged life had induced the production of the submerged 

 type of leaf in the apical region. 



Fig. 94. Callitriche verna, L. Shoot from a 

 ditch near the Cam, May 17, rgri, to show 

 the difference between the submerged and 

 floating leaves. The leaves down to, and 

 including, the pair marked a, a were floating. 

 (Reduced.) [A. A.] 



Fig 95. Hippuris vulgaris, L. 

 Leaf whorls. (| nat. size.) 

 A, water leaves; B-D, air 

 leaves of land form. B and C 

 have fruits in the leaf axils. 

 [After Gliick, H. (191 1), 

 Wasser- und Sumpfgewachse, 

 Bd. Ill, Figs. 23 a-d, p. 250.] 



Among the Umbelliferae, a differentiation between water 

 leaves and aerial leaves is not at all uncommon. There are 

 several instances even among our native plants. Sium latifolium 

 is a very striking case. At the end of May, at Roslyn Pits, Ely, 

 the present writer has seen a quantity of this plant, in a non- 

 flowering condition, bearing three types of leaf all three some- 

 times occurring on a single individual (Fig. 97, p. 149). These 

 were firstly, submerged leaves, either simply-pinnate but 

 deeply incised (Fig. 98, p. 150), or compound-pinnate with 



10- 



