PHYSIOLOGY. 



117 



watery eosin, the phloem portions of the bundles were stained as though 

 the coloring matter had traveled up in them. The whole subject of the 

 course of water in aquatic plants needs investigation. 



Respiration is to be looked upon as causing some of the most charac- 

 teristic structures of water plants, namely the air-canals. In August, 1867, 

 Lechartier experimented on N. alba as it grew naturally in the rivers. 

 He found that on cutting off a leaf just below the water surface a flow of 

 gas takes place from the air-canals of the petiole. This flow continues 

 throughout the day, ceases at dark (7 p. m.), and begins again the next 

 morning (8.30 a. m.). It may continue even against a pressure of 26 cm. 

 of water, and after all of the leaves have been cut off the plant. In one 

 instance 220 cc. was collected from a single petiole in an afternoon, and in 

 another 262 cc. The total amount of gas from a single plant was 1,028 cc. 

 in fifty-three hours. In these plants all of the leaves were submerged 

 during the experiments. A plant with several floating leaves was next 

 used. A petiole was cut off near the leaf, and when held i cm. below the 

 water surface no gas came out ; when raised in a closed inverted tube of 

 water 10 cm. above the surface of the river, the flow amounted to 600 cc. 

 in fifteen minutes. This gas was tested in portions of 60 cc. collected 

 successively, and at three times of day, as follows : 



The first, fifth, and tenth portions of air contained 



From these figures it appears that the air drawn out last at each 

 period (V, X), and therefore probably from the stem and roots of the 

 plant, contains more carbon dioxide than that in the petiole, and that the 

 oxygen content of the inclosed air increases greatly while the plant is 

 exposed to sunlight. At best, though, the proportion of oxygen present 

 is much less than in atmospheric air. It appears also from the first 

 experiments that the gases are not necessarily, if at all, derived directly 

 from the air. Barthelemy (1874) attempted to repeat Lechartier's work, 

 but found that the flow of gas diminished rapidly after the first gush ; he 



