412 Dr. F. F. Blackman and Miss G. L. C. Matthaei. [Apr. 11, 
unexpected.* For succulents, and for them only, have such temperatures been 
previously demonstrated. 
Besides a profound influence on respiration, and hence on assimilation, this 
internal heating up of leaves in bright light may have an important and 
hitherto unrecognised bearing on quite other phenomena, such as the success 
or failure of inoculation of leaves by the germ-tubes of parasitic fungi, etc. 
Section IL].— Assimilation in Natural Illumination. 
We now pass to a consideration of the relation of leaf-temperature and 
intensity of illumination to the observed values of assimilation under various 
natural conditions. 
A brief account of the relation of CO:-assimilation to temperature must he 
given first. 
In “ Assim. and Resp. III” there have been given full data, and also the 
first principles of this relation. A further theoretical consideration of these 
principles will be found in a contemporary article by one of us.| We may 
here sum up as much as will concern us. 
1. For each temperature at which the assimilating cells of a leaf may find 
themselves, there is a particular maximal amount of assimilation possible. 
No increase in the amount of incident light or of available CO. will cause the 
leaf to assimilate more if the temperature remains unchanged. If there is 
not enough COs, or not sufficient light available, then one of these becomes a 
“limiting factor,” and the maximal assimilation for the temperature cannot 
be attained, but some smaller amount only. 
2. The temperature-maxima for assimilation increase rapidly as one ascends 
the temperature scale, the relation between temperature and assimilation 
being very similar to the relation between temperature and respiration. 
3. At moderate temperatures a leaf can maintain its assimilation at the 
maximal value continuously for a considerable time, but at higher tempera- 
tures (towards 30° C. for cherry-laurel) the initial maximal value is not 
maintained ; instead, a regular falling-off sets in. 
4. This decline from the initial value is, for each temperature, more rapid 
at first than subsequently. The higher the temperature the more precipitous 
is the whole declining curve. These phenomena necessitate the introduction 
of a “ time-factor” into assimilation values at high temperatures. 
The added complication of a time-factor makes critical work with varying 
* See note A on p. 459. 
t Askenasy, ‘ Botan. Zeitung,’ 1877. 
{ F. F. Blackman, “ Optima and Limiting Factors,” ‘ Annals of Botany,’ vol. 19, April, 
1905. 
