142 THE BIOLOGY OF FLOWERING PLANTS 



function in question took place most vigorously. The 

 relation of temperature to assimilation, growth, respiration, 

 etc., was commonly expressed by a graph having a cha- 

 racteristic form, with a branch rising to the optimum and 

 another descending from it, and tending to zero at the 

 maximum temperature — at which the process ceased. 

 Now Matthaei showed that the assimilation rate of the 

 cherry laurel, expressed in mgs. CO2 per 50 sq. cm. leaf 

 surface per hour was 2"o at 0*4° C, y6 at 9*2° C, 7 at 

 15° C, 1 0*1 at 23*7° C. The coefficient of increase for a 

 rise of 10° C. is 2*1. For Elodea the coefficient for 10° C, 

 calculated from the observed values at 7° and 13° C, is 2'05. 

 The primary relation between temperature and assimilation 

 is that assimilation increases with rise of temperature, the 

 coefficient for 10° C. being just over 2. This is the relation 

 which Van 't Hoff had shown to hold between rise of tempera- 

 ture and an ordinary chemical reaction. The temperature 

 coefficients are generally much lower for purely physical or 

 photochemical reactions (about i"i). That the coefficient 

 for photosynthesis is so high, again points to the importance 

 of the protoplasmic factor, which must be concerned with 

 chemical reactions. Willstatter found for elm and elder 

 coefficients of about r^. He considers that this lower 

 figure indicates limiting effects by physical processes, such 

 as the rate of diifusion of carbon dioxide. 



At any temperature from 25° C. downwards, the rate of 

 assimilation remains constant for prolonged periods ; suc- 

 cessive hourly determinations during six or seven hours give 

 the same value. But from 30° C. upwards this is not the 

 case. At 30° C. it was found that the rate fell from 157 mg. 

 carbon dioxide per 50 sq. cm. leaf surface for the second 

 hour to i2'o mg. for the fifth hour ; at 37° C. the corre- 

 sponding figures were 237 mg. and 10-9 nig. ; at 40° C. they 

 were 14*9 mg. and 4*8 mg. It will be seen that at 40° C. the 

 fall is more rapid than at 37° C, and at 37° C. more rapid than 

 at 30° C. It is clear that the determination for the second 

 hour — the earliest that could be made in practice — does 

 not give us the initial values for these temperatures. These 



