HYDRILLA VERTICILLATA 9 



plant were spread over the sugar and exposed to the sun. 

 It is probable that the oxygen evolved by the plant under 

 sunlight caused the bleaching action. The quantity of 

 oxygen given out by the plant is considerable ; the water 

 of the pond in which the plant grows is relatively pure, due 

 to the oxidation of organic impurities. I have introduced 

 a number of these plants into an aquarium in which a large 

 number of aquatic animals are living in comfort. The 

 oxygen given out by the Hydrilla is quite sufficient, there 

 being no necessity for further aeration of the water. 



The plants employed for the experiments were growing 

 very vigorously in the three ponds in the grounds of the 

 Institute. One of these tanks is exposed to full sunlight ; 

 the second is in a relatively dark place, and the third is 

 exposed to bright light from the northern sky but not to 

 direct sunlight. The plants grown exposed to the sun were 

 found to be less sensitive than those exposed to the northern 

 sky ; those grown in the shade of trees also exhibited a rela- 

 tive depression of physiological activity. When the plants 

 were brought to the laboratory, the precaution had to be 

 taken to place them in glass or earthen jars. For traces 

 of metal act most injuriously ; experiments carried out on 

 the subject showed that ten parts of copper salts in a 

 million are enough to abolish the photosynthetic activity 

 of the plant. 



The water of the ponds in which the plants grow was 

 analysed and found to give the following composition. 



Table I. — Analysis of iooo c.c. of Pond Water 



Organic solids 0-090 grm. 



Inorganic solids consisting of alkali carbonates 



with traces of chlorides and sulphates . . 0-129 ,, 



Alkaline earth carbonate .... 0-055 •> 



Insoluble oxides and phosphates . . . o-oii ,, 



Total solids in 1000 c.c. of water . . . 0-285 § rm - 



The following table gives the proportions of dissolved 

 nitrogen, oxygen and carbonic acid, the latter consisting 

 of free and combined C0 2 . The proportion of nitrogen to 

 oxygen in the atmosphere is as 4:1; but, on account of 



