Chap. IV] 



THE AIR 



7i 



2. THE AIR CONTAINED IN WATER. 



Air dissolved in water is richer in oxygen and much richer in carbon 

 dioxide than is atmospheric air. Yet the amount of oxygen at the disposal 

 of a plant is smaller in the former medium than in the 

 latter. According to Forel, a liter of water from the 

 surface of the Lake of Geneva contains : — 



■ J.yK- 



Fig. 43. Caulerpa 

 prolifera from an aqua- 

 rium, with excrescences. 

 Natural size. 



As the diffusion of air in water is very slow, whenever 

 the latter remains very still there is a great risk of 

 scarcity of oxygen. Plants growing in still waters are 

 accordingly provided with means for utilizing to the 

 fullest extent the available oxygen, which is not only 

 dissolved in the water but is also produced during the 

 assimilation of carbon dioxide ; on the other hand, 

 in very agitated water, where aeration is much more 

 thorough, plants exhibit such adaptations to a lesser 

 degree. 



The large size of the surface, when compared with 

 the mass, of aquatic plants is evidently closely connected with their demands 

 for oxygen. I learned from my friend and colleague Noll an interesting 

 illustration of this view. He cultivated 

 Caulerpa prolifera in the still water of 

 an aquarium, and thus obtained plants 

 that were quite healthy but most pecu- 

 liarly modified (Fig. 43). The so-called 

 leaves, which under normal conditions 

 are tongue-shaped and entire, in such 

 aquarium-grown individuals fray out 

 into numerous thin segments, so that 

 there is evidently a considerable increase 

 of surface. This difference strikingly 

 recalls those existing between the sub- 

 merged and aerial leaves of many aquatic 

 plants. 



The unfavourable conditions for the 

 supply of oxygen to aquatic plants 



has led to a considerable increase in the development of the air-passages 

 that were already present in the terrestrial ancestors. In aquatic plants 

 the air-passages are spacious tubes (Fig. 44) which conduct the oxygen set 



Fig. 44. Elatine Alsinastrum. Trans- 

 verse section of stem. Magnified. After 

 H. Schenck. 



