118 A. H. Graves, 



thicker than the others, ma}' possibly be attributed to the necessity 

 for some slight degree of iirmness in the covering of the shoot. 



Besides the thinness of the walls, the epidermis of the leaf 

 exhibits the following two remarkable peculiarities, which have 

 alreadj' been observed in similar aquatics by Warming (1902), 

 Schenck (1886), Goebel (1893) and others, and need not, therefore, 

 be entered into in detail here. 



The light is weakened to such an extent by reflection on the 

 surface of the water, absorption in the water, &c., that most of the 

 chloroplasts, for the purpose of the best illumination possible, are 

 located in the epidermal cells, which therefore assume the role of 

 photosynthesis, but yet have not at all the shape of the palisade 

 cells of land plants. 



As in the majority of other submerged plants, no stomata occur 

 in Ruppia, nor, as already ascertained by Sauvageau (1891, II, p. 209) 

 an}' of the apical leaf pores found by him in other water plants, 

 so that openings of any kind are lacking in the epidermal covering. 

 The reasons for this, dependent on the characteristic mode of food 

 absorption, the lack of a transpiration current as it occurs in land 

 plants, the extreme permeability of the leaves of aquatic plants ta 

 gases, &c., have been fully elaborated by the authorities quoted 

 above (Schenck, 1886 and Goebel, 1893) and need not be dwelt 

 upon here. 



It seems to be generally admitted that where stomata do occur 

 in submerged species, they are to be looked upon as hereditary 

 structures, rather than as possessing any ecological significance. 



Schenck (1886, p. 6) claims that stomata in submerged leaves 

 are positively harmful, admitting the water into the air reservoirs 

 located in the lacunae. Sauvageau (1891, II), although admitting their 

 uselessness, maintains that they are not harmful to the plant. They 

 have gradually disappeared from the leaves of water plants, not 

 because they are harmful, but because they are useless. 



Development of slime. The slime developed by the axillary scales 

 in the shoot has been already treated in detail. It is of ecological 

 significance in that a protection is thus efi'ected for the delicate 

 growing points against their aquatic environment, the protective 

 function of slime being well known (cf Goebel, 1893, pp. 232-237). 



b. Development of Air Spaces. 



The formation of large and small intercellular air spaces, most 

 pronounced in stem and leaves, is one of the most striking histologi- 

 cal characters of the shoot system. In general, the larger of these 

 air spaces, such as the zone occurring in the stem, and the two 



