AQUATIC PLANTS. AERENCHYMA 441 



consisting largely of oxygen, escape from the cut surfaces ; this 

 phenomenon has long been utilised by physiologists as a means of 

 demonstrating the process of photosynthesis. In view of the slowness 

 with which the oxygen diffuses into the plant from the surrounding 

 water, it is an advantage, from the respiratory point of view also, if 

 submerged organs are provided with an internal store of oxygenated 

 air ; the possession of such a reservoir of oxygen further renders the 

 plant in some degree independent of fluctuations in the oxygen 

 content of the external medium. Those plants which possess the 

 largest air-spaces will, of course, suffer the smallest amount of 

 inconvenience from the difficulties by which the direct interchange of 

 gases with the surrounding water is attended. 



Air-spaces which are much larger than those of ordinary land- 

 plants, are not restricted in their occurrence to submerged water- 

 plants ; on the contrary, they are also to be found in all the 

 vegetative organs of floating aquatic species, and in the aerial organs 

 of marsh-plants. Unger long ago calculated that air-containing 

 intercellular spaces accounted for as much as 7 1 '3 per cent, of the 

 entire volume of the plant-body of the floating Aroid Pistia texensis, 

 whereas the corresponding values in the case of two terrestrial plants, 

 Brassica Bapa and [the more xerophilous] Begonia manicata, do not 

 amount to more than 17*5 per cent, and 6 per cent, respectively. 

 The aerial organs of such semi-aquatic plants are provided with 

 stomata, and thus agree with land-plants as regards their mode of 

 gaseous interchange. Nevertheless, there are obvious advantages to 

 be gained, in their case also, from the development of large air-spaces. 

 Such plants are liable to be temporarily submerged, or to be exposed 

 to the action of surf ; moreover, their stomata must often be in danger 

 of becoming blocked by water. In the case of floating plants, the 

 air-spaces also serve to increase the power of flotation, by lowering the 

 specific gravity of the plant-body. 



The characteristic large air-spaces of marsh- and water-plants 

 often arise in connection with a special form of parenchymatous 

 tissue, which is termed aerenchyma, 215 in allusion to its principal 

 function. Even the single-layered plates of parenchyma which form 

 the partitions between adjoining air-spaces and -passages in the stems 

 of Scirpus lacustris and of species of Potamogeton, Myriophyllum, and 

 Papyrus, in the fronds of Lemma, in the petioles of Pontederia crassijies, 

 Trapa natans, various Nymphaeaceae, etc., may be included under the 

 head of aerenchyma. There is a certain contrast between this 

 lamellar form of ventilating tissue and the more prevalent stellate or 

 spongy type ; but the two extreme conditions are connected by 

 numerous intermediate stages. Very typical aerenchyma is found in 



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