U2 RESPIRATION 



index of the whole plant is not due to the proportionate 

 increase with age of dead tissue, sclerenchyma and trachea*, 

 for instance. 



Hover and Gustafson * have made the interesting obser- 

 vation that not only does the rate of respiration of the leaves 

 of maize, wheat, oat and sorghum fall off with age, but that 

 there is a gradual increase with increasing age after middle age. 

 This, however, is not shown by the leaves of the sunflower and 

 by the stems of maize. 



Blackman and Parija f have studied the respiration of 

 stored apples. When maturity has been passed the organism, 

 or a part of the organism such as an apple, enters the phase 

 of senescence in which the general tone becomes gradually 

 less owing to the increasing relaxation of the control of what 

 may be termed the protoplasmic factor. One result of this 

 spontaneous change is an increased rate in the hydrolysis 

 of reserve food whereby there is formed more material available 

 for respiration and this leads to a greater production of carbon 

 dioxide. But stored apples are isolated structures, wherefore 

 the only material available for their respiration is that which 

 they themselves contain. It follows that a time will come 

 when starvation begins and this results in a fall in the rate 

 of respiration. This rate at any moment is thus the re- 

 sultant of two opposed factors, increased hydrolysis which 

 accelerates and starvation which hinders. This generalization 

 is based on many experimental observations on apples at a 

 temperature of 22° C. : these show that there is at first a 

 slow increase in the rate of respiration which progresses 

 quickly and then more slowly to the maximum value, after 

 which there is a falling off, slow at first and then faster, 

 towards zero. This sequence is comparable to that observed 

 by Hover and Gustafson mentioned above. Blackman and 

 Parija do not claim that their generalization is fully estab- 

 lished by the evidence produced ; it is, however, a hypothesis 

 which explains the observed facts. 



* Hover and Gustafson: " Journ. Gen. Phys.," 1926, 10, 33. 

 | Blackman and Parija : " Proc. Roy. Soc," B, 1928, 103, 412. 



