[ i63 ] 



CHAPTER XII 



THE ANATOMY OF SUBMERGED LEAVESi 



THE majority of submerged leaves have certain charac- 

 ters in common, the most obvious of which is their 

 deUcacy of structure. On removal from the water they gene- 

 rally collapse rapidly, and in some cases, e.g. Hippuris vulgaris, 

 when they are plunged into alcohol the chlorophyll begins 

 visibly to pass into solution almost from the first moment. The 

 general tenderness of the leaves is due to the thinness of the 

 mesophyll and the absence of differentiation between spongy 

 and palisade parenchyma, and also to the relative lack of me- 

 chanical elements and the slight development of the cuticle^. 

 It is indeed the epidermal characters such as the reduction 

 of cuticle which most markedly distinguish submerged from 

 aerial leaves. 



It will be remembered that, in general, the epidermal cells of 

 the leaves of Dicotyledons tend to be sinuous in outline, while 

 those of Monocotyledons are more rectangular. But in the case 

 of such a plant as Callitriche verna (Fig. 1 1 1, p. 1 70) which has 

 both aerial and submerged leaves, it is found that, though the 

 aerial leaves show the characteristic Dicotyledonous sinuosity 

 in the form of their epidermal cells, the corresponding elements 

 in the submerged leaves have straight walls, and hence approach 

 the Monocotyledonous type. An interesting hypothesis on this 

 subject was put forward long ago by Mer^. He drew attention 

 to the fact that the epidermis was the tissue most directly 

 affected by transpiration, and suggested that variations in that 

 function might exercise an influence upon the form of the 

 epidermal cells. According to his view, when transpiration is 



^ For a comprehensive account of this subject see Schenck, H, (1886), 

 which has been largely drawn upon in the present chapter. 



2 A cuticle, though thin, seems to be invariably present. See Geneau 

 de Lamarhere, L. (1906). 3 jvier, . (1880I). 



II- 



