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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 nbro-vascular bundles are usually few and 



FIG. 139. SECTION OP STEM OF Potamogeton, SHOWING AIE 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 lacunae 

 filled with air occupying considerable space in the distribu- 

 tion of the tissues (fig. 139). Their rigidity is secured by 

 the turgescence of the parenchymatous cells, and buoyancy 

 is much assisted by the air in the lacunae. 



The primary root is generally feebly developed, and, as 



