300 GROWTH 



i 



pressure are due to a complication of causes, among which may 

 be mentioned the change in the pressure of the free oxygen, and 

 turgidity, and unusual conditions offered the transpiratory 

 mechanism. The most recent investigations on this subject by 

 Schaible bring the following conclusions : The growth of shoots 

 is accelerated, and germination is retarded by diminished baro- 

 metric pressure. The retardation of germination is due to the 

 diminished pressure of free oxygen. 1 



379. Influence of Temperature upon Rate of Growth. Fasten 

 the cord of an auxanometer to the tip of a leaf of Narcissus, and 

 record the rate of growth during two days and note whether the 

 rate is increasing or decreasing. Now cover the plant with a bell- 

 jar with a tubulure at the top, through which the auxanometer 

 cord may be passed. Place several cylinders filled with crushed 

 ice and salt in the bell-jar, to lower the temperature, and suspend 

 a thermometer near the organ to which the auxanometer is at- 

 tached. Note temperature at intervals of 30 minutes for 4 hours. 

 Allow the temperature of the air in the bell-jar to rise as the ice 

 melts. On the following day plot the curve of growth for the 

 two days previous to the exposure to low temperature, and com- 



' pare with the curve of the rate made in the cold. How long was 

 necessary for the effects of the low temperature to be shown by 

 the plant ? Note acceleration of growth as the temperature rises. 



380. Age, Senescence and Death. The duration of life of a single 

 individual plant 'may include but a few hours in the case of the 

 simpler forms, or may extend over many centuries in the case of 

 woody trees and shrubs. A cell constituting an individual bac- 

 terium may develop through a period of fifteen minutes or more, 

 then undergo division into two or more separate organisms ter- 

 minating the career of the single individual, but not by the death 

 of living matter. The increasing complexity of the higher organ- 



1 Schaible, F. Physiologische Experimente ueber das Wachstum und die Keimung 

 einiger Pflanze unter verminderten Luftdruck. Beitr. z. Wiss. Bot. 4: 94. 1900. 



Curtis, C. C. Turgidity in mycelia. Bull. Torn Bot. Club. 27 : 1-13. 1900. 



Galloway, T. W. Studies on the cause of the accelerating effect of heat on growth. 

 American Naturalist. 34 : 949. 1900. 



