CHAP. XLII BENTHOS OF LOOSE SOIL 181 



The Leaf. The dependence of the leaf-shape (and partly shoot-form) 

 on the medium is particularly striking. The following are the five types 

 of leaf -shape : 



1. The floating. 



2. The submerged. 



a. The zosteroid. 



b. The elodioid. 



c. The isoetoid. 



d. The myriophylloid. 



The submerged types fall into two groups : that including very finely 

 segmented leaves occurring mainly in Dicotyledones (type 2 d), and that 

 including essentially long and linear leaves (types 2 a, 26, 2C). 1 



i. The floating leaf has already been mentioned as occurring in the 

 hydrocharid-formation. 2 It is found especially in associations thriving in 

 calm inlets or under shelter of reed-swamps. This type is possessed by 

 Nymphaea, Nuphar, Cabomba, Brasenia, Limnanthemum, Hydrocleys, 

 Elisma, Batrachium, Trapa, Callitriche, species of Potamogeton (P. 

 natans), Polygonum amphibium, and other genera with the same general 

 form. The leaf is broad (orbicular, ovate, cordate, reniform, rhombic 

 or elliptic ; rarely lanceolate), undivided and entire, rarely crenate or 

 incised (as in Trapa and Batrachium), comparatively thick and tough 

 (coriaceous), sometimes possessed of a mechanically strengthened or an 

 upwardly bent margin, and excellently adapted to rest on water and 

 resist movements of the latter ; the gigantic floating leaves of Victoria 

 regia, Euryale ferox, and others are, in addition, strengthened by stout 

 ribs on the under-surface. The floating leaf is necessarily adapted 

 to transpire, and thus provides a transition to the land-plant. 



Stomata occur solely or mainly on the upper face, whose epidermis 

 contains no chlorophyll ; they are protected from being plugged with 

 water by the deposition of wax in or on the cuticle, which is thus rendered 

 unwettable. This it is which gives often a glossy or whitish appearance 

 to the upper face. 3 



The lamina of the floating leaf is dorsi-ventral, showing palisade tissue 

 towards the upper face, and very lacunar spongy parenchyma towards the 

 lower face. The lower face is often coloured dark red by erythophyll, 

 the significance of which is not yet known. Prickles on the lower face 

 of the lamina and on the stalk are shown by Victoria and Euryale. 



The petiole of the floating leaf has the power of adjusting itself according 

 to the depth of water, in such a way that its growth ceases when the 

 lamina comes in contact with the atmosphere. In the case of shoots 

 with long internodes, the latter are likewise similarly arrested in growth, 

 as in Trapa and Callitriche ; in such cases, the proportions as regards 

 length between the petioles and insertion of the floating leaves is such 

 that all the blades are accommodated with space on the water. Frank 

 suggested that growth of the petiole is promoted by the pressure 

 of the overlying column of water ; other investigations have shown that 

 contact with the atmosphere and more intense illumination are respon- 

 sible for the shaping of the floating leaf. 4 



1 For the literature, see Schenck, 18866. * See Chapter XL. 



3 See Jahn, 1886. 4 Frank and others ; see literature in Schenck, 18866. 



