1910.] 



Vegetable Assimilatiori and Respiration. 



431 



16"5 milligrammes per hour. The very close agreement of this value with the 

 average result for turgid leaves in the Table is conclusive* 



Such a high rate of assimilation has always been regarded as remarkable. 

 Now that it has been confirmed its significance is profound. 



For instance, such rapid assimilation is only possible with a high leaf- 

 temperature. Blackman and Matthaei have showuf that for any given 

 temperature there is a maximum rate of assimilation which cannot be 

 exceeded unless the temperature is raised. If a moderate correction be 

 made for the assimilation of the carbon dioxide formed in respiration,| the 

 total assimilation becomes equivalent to a gain of about 18 milligrammes of 

 dry substance per hour. Assuming that Blackman and Matthaei's tem- 

 perature assimilation Qurve for Helianthus tuhei-osus'^ holds approximately 

 also for H. annwus\\ it follows that the minimum temperature is between 

 23° and 24° C. 



The temperature recorded by a bright mercury thermometer in the open 

 during the experiments was usually above this temperature ; and all the 

 available evidenced points to the conclusion that the internal temperature 



* The fact that at the end of his experiment Sachs had to reject one half-leaf because 

 it had wilted, and thought it advisable to float the others on water, led me to infer that 

 even these had become somewhat limp ; but from the result which he obtained it seems a 

 necessary inference that the leaves remained turgid till very near the end of the experi- 

 ment, and that Sachs stopped the experiment aslsoon as they began to droop. 



The result obtained by Brown and Morris, lO'O milligrammes, is intermediate between 

 those given by leaves in the second and third categories in the table, i.e., it corresponds to 

 a somewhat limp condition. 



t Matthaei, 'Phil. Trans.,' 1904, B, vol. 197 ; Blackman, F. F., 'Annals of Botany,' 

 1905, vol. 19 ; also Blackman and Matthaei, ' Roy. Soc. Proc.,' 1905, B, vol. 76. 



I See Brown and Escombe, ' Roy. Soc. Proc.,' 1905, B, vol. 76, p. 69. The starch 

 equivalents of the values there given for the respiration of leaves of H. annuus are, at 

 19°"6 C, 0"7 milligramme per hour ; at 31°'2 C, 20 milligrammes per hour. As in my 

 experiments the leaf-temperature was probably never lower than 20°, usually nearer 30^, 

 and may well have exceeded 30^ at times, 1*5 milligramme is a medium estimate of the 

 average rate of respiration. 



§ See loc. cit., 1905, p. 413, and fig. 2 on p. 414. Blackman and Matthaei there record 

 a rate of assimilation at 22°'3 of 13*1 milligrammes CO, per 50 sq. cm. per hour, which is 

 equivalent to 16"1 milligrammes stdrch per sq. decimetre. (On the use of the starch 

 equivalent in calculating the increase of dry weight corresponding to a given intake of 

 CO., see Thodaj', loc. cit., p. 10.) 



II Considering the similar character of the leaves of Helianthus annuus and H. tuberosus 

 it would seem to be a justifiable assumption that, when the supply of carbon dioxide and 

 radiant energy are abundant, their rates of assimilation at any given temperature will 

 be approximately identical. Cf. the curves given by Blackman and Matthaei for such 

 different leaves as H. tuberosus and Cherry Laurel, loc. cit., p. 414. 



H Direct experimental evidence of such differences of temperature between leaves and 

 their surroundings, especially when exposed to sunlight, has been given by Blackman 



