THE RHYTHM OF OXIDATIVE PROCESSES 



279 



this level by about 1 -5 times takes place only by the end of the irradia- 

 tion when the accumulated dose reaches 10,000 r. The droji of free 

 oxygen level down to its mininnnn takes place not earlier than 3 hr 

 after irradiation termination. 



In another experiment with an exposure of 3,000 r, the primary, 

 irregular, but distinct, rhythm is inhibited by the time of termination 

 of the exposure. A complete arrest is observed after one more hour, 

 while after 3 hr the rhythm is gradually recovered. 



In this case both the inhibition of the rhythm and its restoration are 

 observed without any change in the general level of free oxygen content. 

 An impression is gained that the disturbance of rhythmic processes is 

 related to some earlier functions than a change in the total level of 

 oxidative processes and, therefore, of free oxygen content. 



Imin 



Fig. 7. — Change of rhythm of yeast cells as the result of irradiation. Dose, 5,000 r. 



1, before irradiation; 3, 1 hr after irradiation; 



2, 3 min after irradiation; 4, 1-5 hr after irradiation. 



It is notew orthy that this inhibition of rhythmic activity, as well as 

 the change in free oxygen level proceed in a different manner in different 

 parts of a plant. It is particularly characteristic of the growing root. 

 According to available data, the stems or leaves of a plant which possess 

 a higher radiation resistance do not show an immediate depression with 

 lower doses than those mentioned; moreover, with the dose of 2,000 r 

 they show even a change and increase of the rhythm, as if radiation 

 stimulates, and an appearance of a more distinct and regular component 

 of rhythmical activity. 



A very distinct cessation of the rhythmical process can be observed 

 with an intensely gi'owing yeast culture in a liquid medium (Fig. 7). 



