118 A. H. Graves, 
thicker than the others, may possibly be attributed to the necessity 
for some slight degree of firmness 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 
already 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) 
any 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 to 
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 effected for the delicate 
growing points against their aquatic environment, the protective 
function of slime being well known (cf. Goebel, 1898, 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 
