88 PLANT PHYSIOLOGY 



The following table gives an idea of the quantity of carbon 

 dioxide that is liberated by various plant organs during 24 hr. at 

 room temperature per gram of dry weight. 



Cubic Centimeters of 

 Plant Material CO 2 Released in 24 Hr. 



Full-grown leaves 12 to 24 



Growing rootlets 40 to 80 



Buds of trees 35 to 70 



Germinating seeds 60 to 120 



Growing mycelia of fungi 270 to 1 , 800 



With the maturing of plants, the intensity of respiration of 

 its organs decreases sharply. Thus, for instance, Kidd, West, 

 and Briggs (1921) observed that leaves of a sunflower plant 

 22 days old liberated 3 mg. of carbon dioxide per hour per gram 

 of dry weight; 36 days old, 0.81 mg.; 50 days old, 0.46 mg.; 

 64 days old, 0.34 mg. ; 99 days old 0.25 mg.; and 136 days old, 

 0.08 mg. Thus toward the end of the vegetative period, the 

 intensity of respiration decreased to one-fortieth of the initial 

 rate. 



The object of respiration in plants is the liberation of energy 

 for use in vital processes. On account of their enormous surface 

 and a comparatively low rate of respiration, it is impossible for 

 plants to maintain a temperature at a level much higher than 

 that of the surrounding air. But when a considerable quantity 

 of intensely respiring plant material, such as germinating seeds, 

 flowers, or leaves, is put into an insulated container, e.g., a 

 Dewar flask (Fig. 20), a rise in temperature of 30 to 40 or even 

 50°C. may be observed. This extreme temperature may 

 eventually result in death of the plant material. Still more 

 accentuated is the rise of temperature during the respiration of 

 fungi and bacteria. On damp hay, for example, there develops 

 rapidly such a rich flora of microbes that it soon becomes very 

 warm and may ''heat" if it is not scattered. A considerable 

 rise in temperature may be observed also in some large flowers. 

 The flowers of Victoria regia, for instance, may have a tempera- 

 ture 12°C. higher than the surrounding air. In the inflorescence 

 of Arum, Colocasia, and other aroids, this difference may be as 

 high as 30°C. 



Part of the energy liberated during respiration may take the 

 form of light. Phosphorescence, however, is observed only in 



