276 LECTURE XIII. 



of a Tobacco-plant and of a Gourd in a moist soil at a 

 temperature of 3 5C. did not absorb sufficient water to 

 compensate for the loss by transpiration, they did so when 

 the temperature of the soil was raised to 12 18 C. Hein- 

 rich found that the number of bubbles of oxygen given off 

 by Hottonia pahistris (immersed in water and exposed to 

 light) at a temperature of IO'6 ii'2 C. was 145 160 in a 

 unit of time, the number given off at 3iC. was 547 580. 



It must not be assumed, however, that there is an exact 

 proportion between the rise of the temperature and the in- 

 creased activity of any particular metabolic process. The 

 nearest approach to such a proportion is afforded by Mayer's 

 observations (p. 198) on the absorption of oxygen by seed- 

 lings at different temperatures, but even in this case it is not 

 exact. 



The activity of the metabolic processes cannot be indefi- 

 nitely increased by a rise of temperature. As the temperature 

 rises from the zero-point for any one process, the activity of 

 that process is increased until a certain degree of temperature 

 is reached, and any further rise of temperature leads to a 

 diminished activity of the process in question, until, at a 

 certain temperature, it ceases altogether. We see, then, that 

 for each metabolic process there are three cardinal points of 

 temperature ; the minimum- or zero-point at which the per- 

 formance of the process is just possible, the optimum-point 

 at which it is carried on with the greatest activity, the maxi- 

 mum-point at which it is arrested. In our study of respiration 

 we met with two cases which illustrate this statement. In the 

 table of Mayer's observations on seedlings of Tropceolum majus 

 (p. 198), it appears that the absorption of oxygen gradually 

 increased as the temperature was raised from 22-4 C. to 

 35 C., but that it was less considerable at 38-2 C. than at 

 35 C. ; from this it is evident that the optimum degree of 

 temperature is lower than 38*2 C. The other case, which is 

 of a similar nature, but refers to the evolution of carbon 

 dioxide, occurs in the tables of Deherain and Moissan's 

 experiments with leaves (Sinapis alba, p. 199). With regard 

 to the decomposition of carbon dioxide and the evolution of 



