292 PLANT PHYSIOLOGY 



Accompaniments of Respiration; Production of Heat. — There 

 are certain accompaniments of respiration which are normally 

 found and which, in some cases, are of much physiological im- 

 portance. First among these is the production of heat. Not all the 

 energy liberated is used up by the processes of the plant, but a 

 certain amount is freed in the form of heat. Under ordinary condi- 

 tions this heat is dissipated into the surrounding air so that the 

 plant does not have a higher temperature than the environment, 

 but if it is confined in a calorimeter so that very little heat es- 

 capes, it can then be easily measured. The temperature of growing 

 shoots is seldom more than 0.3° C. higher than the surrounding 

 air, but, if plant parts are taken which are extremely active, 

 greater differences can be observed. During germination and 

 during the time of bud unfolding, respiration is especially intense, 

 and at these periods the heat formation can be easily detected. 

 Germinating seeds have been found to give in properly insulated 

 calorimeters a difference in temperature of 20° C. for 100 grams of 

 seed. Seeds used in such experiments must be sterilized or the 

 rise in temperature will be due in large part to the action of bac- 

 teria. This temperature increase, to be sure, is the result of res- 

 piration but respiration of the bacteria rather than that of the 



seeds. 



Bonnier (1893), who carried on extensive researches upon the 



production of heat by germinating seeds, found that the amount 



of heat developed varied with the stage of development of the 



seedling as indicated in the following table : 



Stage of development. Gram calories of heat 



Pea seeds produced per minute 



Soaked seeds 9 



Seedlings — roots 5 mm. long 125 



Seedlings — roots 55 mm. long 75 



Seedlings — stem 20 mm. long 60 



Seedlings— cotyledons withering 22 



Seedlings — cotyledons fallen 6 



As this table shows, the period of greatest heat production is when 

 the roots are about 5 mm. long. Since the evolution of heat is a 

 measure of the excess energy produced over that used, the tem- 

 perature is not an index to the intensity of the respiration but to 

 the inefficiency of the organism at that particular period. Peirce 

 (1912) found that in germinating peas about 8.5 calories of heat 

 per minute per kilogram were given off during the first day. 



In flower clusters differences of 5-10° C. between the tempera- 



