October 2, i914] 



SCIENCE 



491 



plasmolyzed to a moderate degree' recovered 

 in about twenty-four hours. The Spirogyra 

 lives and maintains its normal permeability 

 indefinitely in such dilute sea water. 



When placed in a solution of pure NaOl .4 M 

 the vitality of the ceU is rapidly impaired and 

 it dies in the course of two or three hours.^ 

 If such a cell be plasmolyzed by A M NaCl to 

 a moderate degree' it recovers in the course of 

 half an hour. Since the permeability is in- 

 versely proportional to the time of recovery, it 

 is evident that the NaCl impairs the vitality 

 of the cell and at the same time increases its 

 permeability.^" The two processes go hand in 

 hand. The permeability continues to increase 

 until death occurs, when the cell becomes 

 completely permeable. 



These experiments were repeated with a 

 variety of reagents and were afterward con- 

 firmed in every detail by the method of deter- 

 mining electrical resistance. 



As the result of these and other experiments 

 we may say that the permeability is greatly 

 affected by changes in the composition of the 

 salt solution in which the cell is placed. 

 Normal permeability is best preserved in solu- 

 tions in which the proportions of salts are 

 approximately the same as in sea water and 

 normal vitality is also maintained longest in 

 these solutions. In general we find that vital- 

 ity and permeability are affected in exactly 

 the same way by various kinds of electrolytes. 



This principle may be applied much more 

 generally. The writer finds that all substances 

 (whether organic or inorganic) and all agents 

 (such as excessive light, heat, electric shock, 

 mechanical shock, partial drying, lack of 

 oxygen, etc.) which alter the normal permea- 

 bility of the protoplasm shorten the life of the 



8 This degree is a definite one. It was usually 

 chosen as the condition in which the protoplast just 

 touched the end walls. 



. 9 This applies only to the species of Spirogyra 

 used in these experiments: some species may be 

 more resistant while others are much more sensi- 

 tive and are killed in less than ten minutes. 



10 In other solutions in which vitality is more 

 rapidly impaired the recovery from plasmolysis is 

 also more rapid and we must conclude that the 

 permeability is proportionately increased. 



organism. This is equally true whether the 

 alteration consists in an increase of permeabil- 

 ity, or in a decrease of permeability (followed 

 by an increase) as is the case when certain 

 reagents (such as CaOlj) are applied.^ This 

 is a very striking fact and its significance in 

 the present connection seems to be clear and 

 unmistakable. It shows in the most convincing 

 manner that permeability is a delicate and 

 accurate indicator of vitality. 



"We are unable to say why there is such an 

 intimate connection between vitality and per- 

 meability. It is evident that permeability 

 may control metabolism by regulating the 

 osmosis of various substances, and conversely 

 that metabolism may affect permeability. 

 What is needed is not more speculation in this 

 direction, but a careful analysis of the factors 

 which control permeability. If we are suc- 

 cessful in determining what these factors are 

 we may hope to arrive at a more satisfactory 

 foirmulation, in physico-chemical terms, of our 

 conception of vitality as well as of that of 

 injury. 



W. J. V. 6sTERH0UT 



Laboratory of Plant Physiology, 

 Harvard University 



soil acidity and methods for its detection 



The so-called " acid " soils are peculiar in 

 that a solution obtained by shaking such soils 

 with water will be found, except in rare cases, 

 to be absolutely neutral toward litmus paper. 

 However, if the test paper be brought into 

 direct contact with the soil particles them- 

 selves, a very sharp acid reaction will be ob- 

 tained. These acid soils possess another pecu- 

 liar property in that if shaken with a solution 

 of some neutral salt such as sodium chloride, 

 an appreciable amount of a soluble acid will 

 be found to be set free. 



Two theories have been advanced to explain 

 these properties. The older and perhaps still 

 the most generally accepted theory is the 

 humic acid theory. This theory assumes that 

 there are present in the acid soils, as the result 

 of the decomposition of animal and vegetable 

 matter, some very insoluble organic acids called 

 humic acids. These are supposed to be definite 



