326 



than the upper one, the cells being longer and the walls less 

 distinctly undulating. The lower surface of an upper leaf may 

 often have the same appearance as the upper surface of one 

 placed lower down. 



In CaUitriche as in many other aquatic plants only the 

 upper epidermis is furnished with stomata, except in some 

 cases in emerged leaves, in which they also may be found on 

 the lower (fig. 15 and 16). 



They are numerous, as is to be expected in air-leaves and 

 perhaps also in upper water-leaves, few or none in deeper 

 submerged ones. Sometimes they are deformed. As mentioned 

 above (cf. p. 309) Porsch (1905 p. 84) observed, that the stomata 

 of C. verna behave differently to other stomata, closing if in 

 contact with water and under favourable conditions of light. 



The structure of the chlorenchyma agrees in the main with 

 the drawings and the descriptions of Hegelmaier (1864 p. 31) 

 and ScHENCK (1886 b p. 19). The leaves in some cases are 

 more highly differentiated than those represented by these 

 authors. The palissade-tissue of a plant of C. verna, which 

 is apparently the terrestrial form (Mudderbugten, Greenland ca. 

 70°15' N. lat., N. Hartz Aug. 30.), as well as that of an upper leaf of 

 C. stagnalis (Laugarne, Reykjavik, Iceland, C. H. Ostenfeld Aug. 7, 

 1895), is more developed than in the "land-leaf of C. verna 

 (ScENCK 1. c), while the structure of another leaf, about 6 cm 

 lower down on the stem of the same specimen of C. stagnalis, 

 is almost like the drawing. 



Some linear leaves, deeply emarginate at apex, of C. ha- 

 mulata (Sukkertoppen, Greenland 65°25' N. L., 16. August 1884) 

 are a little more reduced than in any mentioned by these 

 authors. The upper one of the three or only two layers of 

 chlorenchyma cells are more rounded, of an almost isodiame- 

 trical. i. e. more primary shape, only incompletely developed (Fig. 

 17), the intercellular spaces are larger, while the vascular 

 bundle and its starch-sheath are less developed. 



