410 PLANT PHYSIOLOGY 



ture results in increased evaporation from the surface which pro- 

 duces cooling. Any diminution below that temperature causes 

 increased oxidation with a production of heat. Similarly, in main- 

 taining the acidity of the blood, the regulating mechanism is so 

 fine that an increase of one part of H + ion in 10 14 parts of blood 

 causes the respiratory rate to change in such a direction as to 

 restore the normal condition. In plants the responses to gravity, 

 light, etc., are examples of autoregulation (irritability); but the 

 mechanists insist that autoregulatory systems much finer than 

 those of organisms are to be found in the thermostat and other 

 creations of man. Even in inorganic nature a kind of autoregu- 

 lation may be seen in the bed of a river. As the amount of 

 water rises, the width increases, with the result that the depth 

 remains nearly constant as the stream fills the valley. Thus both 

 the flow and depth are regulated. 



Metabolism, from the mechanistic point of view, is purely me- 

 chanical (chemical) . The burning of the sugar in the body is similar 

 to oxidation in a flame. A candle is consequently a fine example 

 of life to the mechanists as well as to the poets. The body of the 

 candle is the stored food. The wick provides a means of trans- 

 locating the digested (melted) food to the point where it can be 

 oxidized in the flame to carbon dioxide and water, releasing en- 

 ergy in the form of heat and light. To be sure, the temperature 

 is higher, but the action of enzymes which bring about oxidation 

 at low temperatures is also purely chemical in nature. 



As to heredity and its effects upon the organism, this is a point 

 which was for a long time hard to duplicate in the inorganic world; 

 but Van Bemmelen (1910) showed that even inorganic materials 

 may vary with their past, i. e., they have a history, which is the 

 essence of heredity. If a silicic acid gel is exposed to air containing 

 various percentages of water vapor, the amount of water absorbed 

 at a given tension of water vapor is not the same if the gel has been 

 exposed to a lower tension as when it has been exposed to a higher 

 tension (Fig. 29). If it has been in a drier atmosphere and the 

 water vapor is now increasing, the curve of water absorption is 

 expressed by A. But if it has been in a moister atmosphere, the 

 amount absorbed is expressed by B. At a vapor tension of 6.3 mm. 

 of mercury the water in the gel when the vapor tension is increas- 

 ing is less than half what it is when the tension is decreasing. 

 Consequently if one places such a gel in a vapor tension of 6.3 mm. 



