Temperature. 77 



obtained by continuing the van't Hoff curve obtained for temperatures 

 below 25°C. As to the second method estimations of the assimilation 

 were made during four successive hours at 30-5"C, ST-S^C and at 

 40*5''C. The values for assimilation so obtained are plotted against 

 time on the same diagram (the abscissje now having a time signi- 

 ficance) in the curves C2 — Cj, D^ — Dj and Eg — Eg respectively, 

 the points Cj, C3, etc. giving the values of assimilation obtained in 

 the experiments. These curves are continued backwards to a point 

 representing zero time, which gives the initial assimilation. The 

 curves are so arranged in the diagram that the position representing 

 zero time in each case is that also representing the temperature of 

 the determinations, so that if the initial values of assimilation are 

 given by the van't Hoff rule, they will fall on the curve drawn on 

 that assumption. This is shown to be the case, so that as Blackman 

 says, there is satisfactory evidence for a preliminary acceptance of 

 the theory that the initial values of assimilation at high temperatures 

 follow the van't Hoff as well as at low temperatures, although above 

 25°C the existence of the ' time factor ' prevents the direct measure- 

 ment of this maximum possible assimilation. The suggested form 

 of the assimilation time curves at still higher temperatures are 

 shown at F and G (Fig. 10). At G the temperature is supposed to 

 be reached at which the assimilation falls at once to zero. 



Experiments conducted by Blackman and Matthaei (1905) in 

 which natural illumination only was employed show that different 

 leaves may have different temperature coefficients for assimilation. 

 Thus whereas Cherry Laurel has a temperature coefficient of about 

 2-\, th&t oi Helianthus tuberosus was found to be about 2'5. The 

 results are summarised in the accompanying diagram (Fig. 11). 



A further point brought out in these researches is that the 

 assimilation rate is affected by the season of the year, thus leaves 

 are more active in February than in April, and from January to 

 March than from October to mid December. These facts are at 

 present unexplained, but they have no effect on the temperature 

 coefficient which is independent of seasonal variation. 



Owing to the more rapid falling off of the assimilation with 

 time the higher the temperature, the temperature at which greatest 

 assimilation is observed will depend upon the time which elapses 

 between the commencement of the experiment and the measurement 

 of the assimilation. Thus in the case of Miss Matthaei's measure- 

 ments the highest value of the assimilation for the first hour after 

 the experiment had run its preliminary \l hours is given at 37-5"C, 



