330 VEGETABLE PHYSIOLOGY 
Some have, however, very short stems which give rise to 
numerous elongated ribbon-like leaves. These flexible 
stems depend for their support upon the nature of the 
medium in which they live, and though they possess a cer- 
tain. rigidity, this is not associated with any great develop- 
ment of woody tissue. Generally the latter is reduced to a 
minimum ; the fibro-vascular bundles are usually few and 
eenscors 
C) 
=—# 
Fic. 189.—SEction or Stem or Potamogeton, sHowinc AIR PassaGEs 
= IN THE CORTEX, 
contain few lignified elements. Their substance is largely 
parenchymatous, and the cells have thin walls. The inter- 
cellular space system is often very complex, large lacunz 
filled with air occupying considerable space in the distribu- 
tion of the tissues (fig. 189). Their rigidity is secured by 
the turgesvence of the parenchymatous cells, and buoyancy 
is much assisted by the air in the lacune. 
The primary root is generally feebly developed, and, as 
