152 Ingvar Jorgensen and Walter Stiles. 
obtained by continuing the van’t Hoff curve obtained for temperatures 
below 25 n C. As to the second method estimations of the assimilation 
were made during four successive hours at 30 , 5°C, 37*5"C and at 
40-5"C. The values for assimilation so obtained are plotted against 
time on the same diagram (the abscissae now having a time signi¬ 
ficance) in the curves C 2 —C 5 , D 2 —D 5 and E 2 —E 5 respectively, 
the points C 2 , C 3 , 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 n 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 
2-1, that of 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 1£ hours is given at 37-5°C, 
