348 POPULAR SCIENCE MONTHLY 



carbon dioxide through the intermedium of the water. This readily 

 absorbs large quantities of the gas. But the percentage existing in the 

 air is so small, the absorbing surface of the lake is so restricted, and 

 the means of transport are so poor that the lake is quite unable to take 

 from the air enough carbon dioxide to maintain a vigorous growth of 

 plants. The lake is forced to depend on its own resources to a large 

 degree for this plant food. Fortunately, these resources are consid- 

 erable. Great amounts of carbon dioxide are manufactured in the lake 

 and these may be utilized as food by the green plants. Thus there is 

 kept up in the lake a sort of internal circulation of carbon dioxide; 

 the stock of the circulating medium being increased and replenished 

 by additions from outside. The activities of animals and the processes 

 of decomposition liberate the gas, which is taken up and manufactured 

 by the plants into organic substances ; and these in turn serve as food 

 and as material for new decomposition; while from the air the water 

 may be absorbing new supplies of carbon dioxide to make good the 

 losses of this process. Thus under normal conditions, the lake would 

 return little or no carbon dioxide to the atmosphere, but would utilize 

 within itself all that it manufactured or absorbed, at least until the 

 plant life became so abundant as to be limited by other causes than 

 that of food supply. 



If this were all, the story would be quite simple and quite to the 

 advantage of the lake. But it is by no means all the story; on the 

 other hand, so far from being forced to solve problems associated with 

 an oversupply of carbon dioxide, the lake has to encounter many diffi- 

 culties in securing an adequate supply of that gas, and is able to meet 

 them only very partially and imperfectly. Since the plants are able to 

 utilize carbon dioxide in the manufacture of starch only during the 

 hours of sunlight, considerable quantities may escape into the atmos- 

 phere during the night. But this is not the only disadvantage as 

 regards the supply of carbon dioxide, with which the plants of the 

 upper water have to contend. By no means all, or even the greater 

 part of the organic matter which they manufacture decomposes in the 

 upper, warmer stratum of the lake. As the plants and animals die, 

 they sink into the lower and cooler water before any great part of the 

 decomposition has been completed. The carbon dioxide which is there 

 produced is discharged into this bottom water. It can not be used 

 there by plants on account of lack of light. The same imperfections 

 of transportation which prevent the access of oxygen to the cooler water 

 in summer make it impossible to transport the carbon dioxide produced 

 there to the upper stratum, where it can be utilized. In certain lakes, 

 indeed, a small portion of this gas may be used in the cooler water, as 

 I indicated above, but, in general, the upper water, as a result of this 

 process, is growing poorer during the summer in the materials on 



