RESPIRATION 303 



convex growing side over the concave side during the time of the 

 geotropic response. That stimulation is accompanied by increased 

 respiration, is also not surprising; wherever there is more activity, 

 more energy is required, with the result that respiration is increased. 



Rate of Growth and Maturity. — From what has been said in the 

 previous paragraph, it is to be expected that respiration varies 

 with the growth rate. The curve of growth is an S-shaped curve 

 (Chap. XXVI) which indicates that growth starts out very slowly, 

 rapidly increases during the early stages of development, and then 

 slows down as the plant matures. The consensus of opinion seems 

 to indicate that the respiration curve carried through the life of 

 the plant runs more or less parallel to this curve, with the result 

 that the grand curve of growth and the grand curve of respiration, as 

 measured both by the amount of oxygen consumed and the amount 

 of carbon dioxide given off, are very similar. Although there are 

 many irregularities (some of which will be discussed later), respira- 

 tion, in general, increases as the plant continues to grow, until 

 after the grand period of growth; then, as growth slows down, 

 respiration also decreases, although there may be a lag in the 

 respiration curve, as reported by Gustafson for tomatoes. Aging 

 tissues generally show a decrease in respiration as found in sugar 

 cane by Bonazzi (1931) and in many other plants by Hee. 



The plant, as a whole, at the time of the maturation of the 

 fruit, has reached the period when respiration is decreasing; but 

 the fruits, during maturation, commonly show an increase in 

 respiration owing to the increase in the sugar content. Gustafson 

 (1929) reports an increase in the ripening of tomatoes, but attrib- 

 utes it to the decrease in pH value occurring at that time, changes 

 in active acidity being more pronounced in ripening tomato fruits 

 than changes in sugar content. This increase in respiration during 

 the maturation of the fruits Gustafson seemed to consider as 

 exceptional, but in the light of the above discussion, we see that 

 this is what might be expected to occur. 



The respiration quotient or coefficient decreases until past 

 middle age as the plant grows; the fraction becomes less than 

 one, which means that the plant takes in more free oxygen than 

 it gives off in the carbon dioxide. The explanation for this is 

 thought to rest upon the fact that cellulose and other products 

 which depend upon the assimilation of large amounts of oxygen 

 are accumulating in the plant. 



