300 PLANT PHYSIOLOGY 



that this is no exception to the rule that increased respiration 

 parallels increased cellular activity. 



Oxygen and Aeration. — Since the amount of oxygen in the sur- 

 rounding air is related to the ease with which it is taken up by 

 the respiratory enzymes, the oxygen partial pressure has an in- 

 fluence. The more oxygen the greater the oxidation if oxygen 

 has been a limiting factor. If it is not, as emphasized by Hee 

 (1930), then increasing the oxygen supply does not increase the 

 respiration. In accord with the law of limiting factors, this is 

 what we should expect. 



Since roots, like all other living organs, require oxygen for 

 respiration, the importance of good soil aeration is obvious. Clark 

 and Shive (1932) found that a continuous aeration of nutrient 

 solutions containing tomatoes, increased the size and weight of 

 tops and roots 1.5-2.76 times those of nonaerated controls. The 

 poor aeration may harm the plant either (1) by permitting a defi- 

 ciency in oxygen (the less usual case), or (2) by poisoning the plant 

 with an excess of carbon dioxide. Thus in buckwheat cultures, if 

 nitrogen is passed through the culture solution instead of oxygen, 

 the plants will survive for some time (Free, 1917), but if carbon 

 dioxide is passed through the solution, the plants soon wilt and 

 die. This is of especial importance in the soil, where great quan- 

 tities of carbon dioxide are being added through the activity of 

 soil organisms. Here this lack of aeration may not only check 

 normal respiration and growth, but it may also render the plant 

 more subject to disease and more susceptible to the attacks of 

 harmful fungi. 



Toxins and Anaesthetics. — Various toxic substances, among 

 which may be mentioned alkaloids, glucosides, formalin, alcohols, 

 and anaesthetics such as ether, chloroform, etc., have been studied 

 in their effect upon respiration. In general, the effect of these 

 materials is to cause a stimulation of respiration at the outset 

 followed by a decrease after a short time. As the dose increases, 

 the stimulating effect lasts a progressively shorter time or may 

 be absent entirely, while the decrease in respiration becomes very 

 noticeable as it rapidly falls off to zero. 



Concentration of the Nutrient Solution. — According to Palladin 

 (1902) the concentration of the nutrient solution has a marked 

 effect upon the respiration rate. The more concentrated the solu- 

 tion the less the respiration, as the following table shows : 



