293 
Blackman. — Optima and Limiting Factors. 
he has not our clue to harmonize them with his own results. The interpre- 
tation here provided harmonizes both sets of observations as examples of 
the effects of contrasted limiting factors. 
Internal factors, such as rate of translocation of plastic material, may no 
doubt play the part of limiting factors, but clear examples of this correlation 
are not to hand. One cannot, however, help suspecting that insufficiency 
of plastic material may play some part in the falling off of growth at 
h ; gh temperatures. As the temperature rises above the ‘ optimum ’ 
for growth, respiration goes on still increasing enormously, and large 
quantities of carbon are compulsorily lost to the growing plant in this way. 
Presumably in some cases, possibly quite generally, the insufficient residue 
of available plastic material in the vigorously respiring part would limit 
growth. 
To take the hypothetical case that translocation could just bring in, per 
unit-time, enough carbonaceous material for the growth at the optimal 
temperature plus the respiration at the same temperature. Then, as 
the temperature rose further and the respiration increased faster and faster , 
so necessarily there would be less and less carbon material available 
for growth. The falling curve of growth would become the complement 
of the rising curve of respiration, and it is of interest that to a large 
extent, whatever be its significance, the two curves actually have this 
appearance. 
To conclude this section on limiting factors, it seems to me instructive 
to point out that in the equality of all conditions except one — which is the 
essence of the £ control-experiment 5 method of investigation — may lurk 
a dangerous pitfall if either of the equalized conditions becomes a limiting 
factor in the result. 
Suppose that it were proposed to test the effect upon assimilation of 
some specific factor that should have an augmenting effect. What more 
natural than to place two similar leaves side by side under similar medium 
conditions for assimilation, one subject to this factor and the other not ? 
This would be the typical ‘ control experiment.’ Yet the assimilation 
of the two leaves might be equally limited by the small amount of carbon 
dioxide in the air, or, if this were augmented, by the moderate light or 
by the low temperature ; thus equal assimilations might be obtained, and 
a negative result announced though the specific factor might really show an 
augmenting effect were another factor not limiting the assimilation. 
III. 
In conclusion, it is desirable to make some reference to a particular 
class of agents that complicate the problems of the magnitude of metabolic 
phenomena which we have been seeking to simplify by analysis. 
