286 
Blackman . — Optima and Limiting Factors. 
coefficients of temperature and of time, this schema may possibly exhibit the 
hypothetical primary relation of all metabolic processes whatever to tem- 
perature. 
It has been mentioned that before taking the assimilation estimations 
at high temperatures a f preliminary ’ of one and a half hours was allowed 
after the initial heating up to the experimental temperature. Hardly any 
investigators have allowed a shorter preliminary time, so that it is clear that 
published values, for physiological processes generally, at high temperatures 
are too low, and special experiments designed to get trustworthy estimations 
as near the initial effect as possible should give higher values. 
Now it is most important to note that not only would the value at each 
high temperature be thus increased, but that these values would be increased 
in different ratios. 
Such special experiments would give values — to take the case of 
assimilation — that were no higher for temperatures up to if C., slightly 
higher values for 30-5° C., and very much higher for 40-5° C. The important 
consequence follows that the observed ‘ optimum ’ temperature will be 
raised by such special procedure. Compare, for instance, the two extreme 
cases on the diagram, in one of which the investigator may be supposed to 
have taken our set of actual first readings, i. e. those at one and a half hours 
after heating up, B, C 2 y D 2 > E 2 , and in the other to have allowed four 
and a half hours’ preliminary and so to have obtained our set of fourth 
readings, B, C 5) D 5 , E 5 . 
The first set 1 will give the ‘ optimum ’ at 37-5° C. with a steep falling 
off to 40-5° C., while the other set will give an optimum at 30-5°C. with 
a gentle falling off to 37-5° C. A more adventurous investigator whose 
method would work with only a quarter of an hour preliminary should get 
an experimental optimum over 40°C., and possibly so close to the extinction 
point that he would decide that a real optimum was absent. Now it 
is in this contradictory state that knowledge really stands as regards 
the relation of respiration to temperature. Clausen 2 has recorded a well- 
marked optimum for a variety of plants at 40° C., while Kreusler 3 finds for 
Rubus no optimum, certainly up to 46° C. 
It is not to be supposed that these differences in regard to respiration 
optima depend on different lengths of actual preliminaries, rather no doubt 
they are to a greater extent dependent on different degrees of high- 
temperature-impermanence of the function with different plants. 
Physico-chemical finality is not to be attained in this matter, but 
special research might at least show how far the recorded optima for 
assimilation and respiration are real metabolic truths and how far they 
are illusions of experimentation. 
1 See numbers in footnote, p. 284. 2 loc. cit. 3 Landwirthschaftliche Jahrbiicher, 1887. 
