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 



Fio. 17. — ^Transverse Section oe Submerged Stem of Water-Violet 

 (Hottonia palustris). (Highly 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 La 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. 



