444 VENTILATING SYSTEM 



quite pervious to air, they oppose, as Goebel has pointed out, a very 

 considerable resistance to the passage of water. It is found, in fact, 

 that where diaphragms are present, the air-spaces cannot be flooded 

 with water, except by prolonged injection at a high pressure. When, 

 therefore, a plant which is provided with diaphragms dies off at one 

 end, or suffers some mechanical injury, flooding of the whole ventilating 

 system is effectually prevented, since water will not readily penetrate 

 beyond the first intact diaphragms. The chambered structure of the 

 ventilating system in the inflated petioles of Pontederia crassipes and 

 Trapa natans, in the fronds of Lemna, etc., should evidently be inter- 

 preted from the same point of view ; but here, also, the walls of the 

 water-tight compartments possess an accessory mechanical significance, 

 just like the majority of diaphragms. 



Aquatic plants which contain few or no diaphragms {Rhizophora, 

 Pilularia, Nymphacaceae, Aroideae, etc.), develop so-called intercellular 

 or internal hairs 217 in their air-passages. The fact that these structures 

 appear to take the place of diaphragms, in itself suggests that they 

 perform a mechanical function ; this view of their physiological signifi- 

 cance is further supported by their arrangement, by their thick-walled 

 character and by their shape, which is most frequently that of an 

 H or X. The polyhedral air-passages in the petioles and peduncles of 

 Nymphaea contain stellate hairs, which project from individual members 

 of the cell-rows that occupy the narrow faces of the polyhedra ; the 

 thickened walls of these hairs are furnished with blunt external 

 protuberances owing to the inclusion of numerous small crystals of 

 calcium oxalate. In the vertical direction these hairs succeed one 

 another at comparatively short intervals. The lamellar parenchyma in 

 certain Aroids (Monstcra, Tornelia, Hcteropsis, Pothos, etc.) is traversed 

 by intercelluar hairs which resemble bast fibres in appearance ; they 

 are branched in A r arious ways, and are often hooked at their ends. 

 The mechanical effect is probably similar to that of felted stellate 

 tissue, such as is found in Scirpus. The lower (proximal) portions 

 of the breathing roots of Sonneratia acida, which extend horizontally 

 beneath the mud, are provided with aerenchyma containing character- 

 istic slightly curved or S-shaped internal hairs. Westermaier 218 

 attributes a very curious function to these structures. He points out 

 that the breathing roots of this mangrove are exposed to a varying 

 pressure according to the state of the tides. When the water is 

 high, the aerenchyma is compressed, and air rich in carbon dioxide is 

 forced out. When the tide falls, the pressure upon the root diminishes; 

 the aerenchyma expands once more, owing to the elasticity of the 

 internal hairs which thus act as springs and oxygenated air is 

 sucked into the intercellular spaces. Westermaier's account implies 



