48 BRITISH PLANTS 
passages of the leaf-stalks, stems, and roots, and so a 
free circulation of air, and therefore of oxygen, is possible 
throughout all parts of the plant. In xerophytes the 
great danger threatening the plants is lack of water ; 
in aquatics a danger equally serious threatens—scarcity 
of air. The aquatic might experience a difficulty in 
obtaining sufficient oxygen for respiration after the 
cessation of photosynthesis if all that formed during 
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Fic. 17.—TRANSVERSE SECTION OF SUBMERGED STEM OF WATER-VIOLET 
(Hottonia palustris). (HiaHty MaGNIFiED.) 
a, epidermis, without cuticle ; b, air-space ; c, woody part (xylem) of vascular 
system. 
the latter process were allowed to escape. Much of it 
does escape into the water, but sufficient is always 
retained in the air-spaces to insure efficient aeration. 
The presence of air in the tissues may also be useful in 
another way. It serves to keep the assimilating organs 
at or near the surface of the water, where oxygen and 
light are most abundant. 
2. The cuticle in aquatics is thin and devoid of wax. 
Water can therefore be absorbed over the whole surface, 
