426 Dr. F, F. Blackman and Miss G. L: ©. Matthaei? [Apr. 11! 
Table [Experiment VIL 




1.. a: | Ba 420 [IST o§; 6. Ter 
ie Difference |, ..Leafs . |: i ae, ye rae 
Middle | between | temperature Average | sched algal a ai 
Se pone tal caleulated | calculated _| observed. Observed fg 1 te Lay Differ 
Wee aera and | from assimi- leaf- a assimila-. temperature. ence, 
Gon observed lation. . | tempera-:} — tion. "(data ea and 6. 
* |temperatures,| (data on ture. . 413 +2 ee gee 
4 minus 3. |  p. 418). Behe) ee 
fo} ° O° . 
9,35 —0°3. 25°3 25 °0 - 0:0117 0 °0115 ee 
10.5 —0°2 0 21 °6 21 *4 0 0095 0 0094: + 1 
10.35 +0°9 25 ‘0 25 °9 0 °0115 0 °0120 — 5. 
) 11.5 +1°2 25 °0 26 °2 00115 0 0122 — 7 
| 11.85 +1°3 21°0 22 °3 0 0091 0 0099 - 8 
| 12.5 +0°8 20°5 21°3 0 -0088 0 0093 i & 
, 12.85 +2°5 18 °1 20 °6 0 0076 - 0 :0088 -—12 





In the last reading the sky is heavily overcast, the excess leaf-temperature 
and the observed assimilation the smallest recorded. The light is obviously 
the limiting factor, and the assimilation well below the maximum. 
In the other readings the observed assimilation is close to that calculated 
as maximal from the observed leaf-temperature. The table is designed to 
show how closely one can calculate the leaf-temperature from the observed 
assimilation, viz., Column 3 from Column 5, the error being given in Column 2; 
and also, inversely, calculate the assimilation from the temperature, viz., 
Column 6 from Column 4, the error being in Column 7. | 
The temperature error (omitting the last reading) averages only 0°8 C., and 
the assimilation error 0°0005 gramme COs. It is to be noted that there are 
no “preliminaries” between the different readings, so that the effects here 
tend to run into one another. 
The relation of temperature and assimilation is exhibited as an extra- 
ordinarily constant one when one recalls that the temperature maxima used 
in the calculation are derived from experiments made at a different season of 
different years, and with artificial light. 
Section 1V.—Jllumination and Temperature as “ Limiting Factors” in 
Assimilation. 
We now pass to the consideration of some experiments which are not 
merely records of the hourly march of natural illumination and temperature, 
but in which the conditions are manipulated to bring out certain points. _ 
In Experiment VIII the leaf was for the first three readings exposed only 
to the diffuse light of a brilliant cloudless day.. The sun’s direct rays were 
throughout intercepted by the shadow-board, and the readings show the slow 
