334 



VEGETABLE PHYSIOLOGY 



growing in ordinary soil. Gaseous absorption takes place 

 through the general surface to a large extent, but this 

 direct supply is insufficient for respiration. The ordi- 

 nary arrangements for aeration, consisting of a network 

 of intercellular spaces freely in communication with nu- 

 merous stomata, are not exhibited by plants surrounded by 

 water. We have seen that many of them have no stomata, 



FIG. 14-2. SECTION OF RHIZOME OF Mar&ilea. 

 .) lacunae in cortex. 



the leaves being quite submerged ; others have relatively 

 few on the upper surfaces of the floating leaves. The 

 gaseous interchange between the interior and the exterior 

 is consequently greatly impeded. The large intercellular 

 lacunae form a- mechanism by which this difficulty is 

 surmounted, affording large reservoirs of air in the interior 

 of all parts that are submerged, so that the slow rate of 

 renewal of air does not impede the gaseous interchanges 



