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, 
Fic. 142.—Srctron or Rarzomre or Marsilea. 
co.la., lacune 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 
lacune 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 
