306 EFI-ECT OF CHEMICAL AGENTS [Ch. XI 



mercury could be poured into the tube. The column of mercury produced 

 the increased pressure in the flask, and the difierence in height of the 

 mercury in the two arms of the tube was a measure of this pressure. To 

 get a pressure below the normal a partial vacuum was produced by a water- 

 pump and the flask was then sealed. We assume (with Beet, 78, p. 1153) 

 that the chief effect of the variation in the atmospheric pressure was the 

 -variation in the amount of oxygen absorbed by the water. Pure oxygen 

 was also used in the flask. 



At a pressure of three atmospheres no growth occurred. At 

 a pressure of two atmospheres growth was slower than at the 

 normal pressure. At three-fourths of an atmosphere also 

 growth was retarded and at one-half an atmosphere death 

 generally occurred. Thus the optimum condition of oxygen 

 tension is near the normal for the atmosphere. 



The same thing is indicated in a qualitative way by the 

 experiments of Loeb ('92). The stems of the hydroid Tubu- 

 laria possess in ordinary water a high regenerative capacity, 

 but in water deprived of oxygen by boiling no regeneration 

 takes place, although, after the water has been aerated again by 

 shaking, rapid growth occurs. 



Hydrogen. — This element forms, in its various compounds, 

 between 5% and 10%, by weight, of the dry substance of 

 organisms. How is it acquired? In the case of plants it is 

 believed that it is taken into the organism as a constituent of 

 water, which combines with carbon or carbonic acid in the plant, 

 forming either starch directly or some other compound from 

 which starch is later derived. Other possible sources of 

 hydrogen are ammonia and its coijipounds, also the organic 

 compounds absorbed. The hydrogen of fungi and animals has 

 clearly been derived from the latter compounds alone. The 

 effect of hydrogen gas upon growth seems to be merely that 

 due to the replacement of oxygen; it has no active effect. 



Carbon is the largest constituent of dry organic matter, of 

 which it forms between about 44% and 60%. In green plants 

 it is obtained for the most part from the carbon dioxide or 

 carbonic acid of the air which is absorbed by the leaves. Other 

 sources of carbon for green plants are found in many organic 

 compounds, such as urea, uric acid, hippuric acid, glycocoll, 

 kreatin, guanin, asparagin, lucin, tyrosin, and acetamid. These 

 afford nitrogen also. Certain green plants make use of animal 



