WATER AND MARSH PLANTS 



373 



woody tissues are so poorly developed that many water- 

 plants collapse when taken out of the water, showing how 

 dependent they are on water for mechanical support. 



The position and mode of growth of the underground 

 stems and roots, even in the same species, vary considerably, 



Fig. 242. Water Buttercup. 

 a, Submerged leaves; b, floating leaves. 



Fig. 243. Transverse Section of Leaf of Flowering 

 Rush, showing large Air-spaces. 



and are determined largely by ' water-content ' in the soil, 

 and the supply of oxygen. The rhizomes and roots are 

 often placed more or less horizontally, and in this position 

 are near to the air-supply. As the soil becomes drier and 

 better aerated, the roots take a more vertical course. 

 The stems and leaves of water-plants are often slimy, 



