330 PLANT PHYSIOLOGY 



the protoplasm is rich in water. Thus seeds of the sunflower (Heli- 

 anthus annuus) have withstood heating for a short time to 140° C. 

 or even to 145° if heated gradually ; and dry spores and some seeds 

 can endure 125° C. for five minutes and over two weeks in liquid 

 oxygen at —210°, making a total range of more than 300°. Bec- 

 querel even reports (1925) that the germination of wheat, mustard, 

 and lucerne seed was unimpaired after these seeds had been for 

 over ten hours in liquid helium ( — 269° C.) ! 



In order to bring about a definite degree of development, to 

 cause the ripening of fruits, for example, a certain amount of 

 heat must be supplied. Various investigators have tried to show 

 that the plants of the same species require a definite number of 

 heat units to reach the same state of development; and while it 

 is true that plants grown at the northern and southern limits of 

 their range require a minimum number of heat units, there is 

 much variation, and in estimating the number of units necessary 

 for the development of plants, one must consider not only the 

 external conditions but the internal life of the plant. The 

 resting period may continue in spite of external conditions 

 favorable to growth, so that it is impossible to prescribe a cer- 

 tain number of heat units and always expect to get the same 

 results. 



Excessive heat promotes chemical changes which are helpful 

 in development, but it may also promote changes which are harm- 

 ful. Constantly increasing the temperature is beneficial only up 

 to a certain point, when the harmful effects more than counter- 

 balance the good. This results in the optimum that has already 

 been observed. The damage done by high temperatures (as men- 

 tioned above) depends to a large extent upon the amount of water 

 present, and for this reason dry organs can withstand much higher 

 temperatures than those actively growing and rich in water. 

 Wheat, for example, when the water content was 10%, was killed 

 at 78° and corn at 68° C. (Robbins and Petsch, 1932); but when 

 the water content reached 30%, the wheat died at about 53° 

 and corn at about 55°. The temperature coefficient of killing by 

 heat is of the same order as that for the denaturation of proteins 

 (Collander, 1924), which indicates that this is the chief source of 

 danger from high temperatures. This is supported by the fact 

 that guard cells of Zebrina and Rumex are killed in water at 62° 

 in about one minute if the stomata are closed but not if they are 



