288 Blackman. — Optima and Limiting Factors. 
Admitting, then, the existence of a ‘ real optimum,’ not to be broken 
down by the most ideal experimentation, we have to seek a clue to its 
causation. 
In vitro , a few chemical processes are known which take place more 
slowly as temperature rises, but it is agreed that this abnormality is not 
a primary effect of the temperature, but that secondary causes are at work. 
The rate of hydrolytic action of isolated enzymes, however, always 
shows a marked optimum temperature effect. Thus Kjeldahl 1 showed 
that malt-diastase hydrolysed increasing quantities of starch up to about 
63° C., after which the action fell off quickly, becoming nil at 86° C. The 
interpretation of this is quite simple, and the diminished effect at super- 
optimal temperatures is due to an actual destruction of the enzyme by the 
heat. It has been clearly proved that the destruction is the faster the 
higher the temperature, and that the apparent optimum is the effect 
of the two opposed processes at work. If the supply of enzyme could 
be kept up constantly to counterbalance its destruction, then the true rate 
of its hydrolytic action at high temperatures could be determined. This 
result could be also indirectly arrived at by the method adopted for 
calculating the initial values of assimilation, but to my knowledge neither of 
these experiments has yet been carried out. 
Should not the optimum for growth be interpreted in some similar 
way ? I think we may regard it as certain that it is due to some secondary 
cause working against the causes that have made for increasing growth 
as the temperature has risen from o°C., and not due to failure of these 
causes as a direct effect of the increased temperature. 
It is possible that the destruction of an enzyme, or the action of some 
anti-enzyme, plays a part in causing the ‘ real optimum ’ which growth 
exhibits, but in the absence of any analytical study of the behaviour 
of growth at high temperatures it is idle to put forward any explanatory 
hypothesis. 
Growth is indeed the finished product of the metabolic loom, and, 
in order that the weaving of the specific pattern of the individual plant shall 
continue for a measurable time at the racing speed of metabolism which 
high temperature enforces, extraordinary co-ordination is required. 
It may well be that, soon, co-ordination will fail in some question of 
supply of material, and it is significant that high temperature does not 
distort the specific pattern of the plant as do negative or positive extremes 
of light or of moisture ; the machinery slackens and the output is less, but 
the pattern is the same. 
It is therefore easy to conceive that falling off of growth-rate may be 
due to a variety of causes, and what is really required is a careful investiga- 
1 Compt. rend. Carlsberg Lab., 1879; cf. Czapek, Biochemie der Pflanzen, Bd. 1, p. 345. 
