266 



THE MAINTENANCE OF THE INDIVIDUAL 



siderably influenced by external conditions. Low temperatures slow 

 up the process as do very high temperatures, there being an optimum 

 temperature for each organism at which the rate of respiration goes on 

 best. Seeds have survived a temperature of —250° C. Experiments 

 with leaves show that the respiratory rate increases rapidly from 

 0°-40° C, from which point it falls slowly until the death of the 

 organism. The amount of food present in the plant is a second 

 factor influencing the rate of respiration, while the rate also varies 

 with the amount of protoplasm in the cells. Light usually increases 

 the respiratory rate, probably because of a parallel increase in food 

 and temperature. It is also found that wave lengths which increase 

 photosynthesis also increase the respiratory rate. Finally, the rate 

 of respiration is greatly affected by poisons or anesthetics, at first 

 being increased, but later slowing down rapidly. In brief, respiration 

 in plants, as in animals, is induced by the action of enzymes, and 

 results in the release of energy. 



Transpiration 



If a healthy potted plant is placed in a dry bell jar and left in the 

 sun for a few minutes, drops of water are seen to gather on the inside 

 of the jar. By covering the pot with a rubber tissue to exclude the 



large. 

 battery 

 jar-. 



(Jovcrect 

 vith 5heet 

 rubbe'T.... 



moifture 



star-t- 



24 Viours later 



Experiment to show transpiration. Read your text and explain what has 



happened. 



possibility of the evaporation of water through its surface and return- , 

 ing it to the jar under similar conditions, drops of moisture are again 

 found after a time on the inner surface of the jar. Obviously, water 

 must come out through the leaves or stem of the plant, a fact which 

 can be demonstrated by weighing it before placing it in the jar, and 



