Blackman . — Optima and Limiting Factors. 287 
A further experimental complication lies in the rate of heating up, 
which must depend partly on the method and partly on the conductivity 
of the organ investigated. If heating up is slow, then the falling off pro- 
duced by passing slowly through the temperatures 40°-44°C. would lower 
the value obtained for 45 0 C., to take a concrete example. 
The optimum has by some investigators been regarded as the highest 
temperature which can be permanently sustained without depression of 
function, but more usually a real optimum is held to be characterized 
by this, that the retardation produced by exposure to super-optimal 
temperature must not be of the nature of permanent injury, and that there- 
fore on cooling again to the optimum temperature there must be a return 
of the function to its highest value. 
There has been little attempt to apply this principle experimentally, 
and it looks as if everything would depend on the time of exposure to the 
super-optimal temperature. Rather than by direct experiment, it is prob- 
able that the high transient values will in future have to be estimated 
by the convergence of the lines of evidence that we have already indicated. 
Only respiration and assimilation have been yet mentioned, and they 
are conditioned by comparatively simple factors, or rather by factors which 
can be kept under control so that these two processes might be expected to 
show the primary relation to temperature not greatly masked. 
In the case of such a complex process as growth one cannot start analysis 
with any such expectation. 
The available published data as to growth are of very little use for our 
inquiry. Those that deal with a wide range of temperatures mostly consist 
of single readings, and these after a long preliminary — in one classical set 
as long as forty-eight hours (Koppen) 1 ; the more detailed studies by 
Askenasy 2 and Godlewski 3 are concerned only with medium temperatures : 
True 4 has a couple of not very significant experiments at a ‘ super-optimal ’ 
temperature. It is, however, the universally held opinion that growth 
exhibits a well-marked optimum in its temperature relation. This optimum 
is placed by Sachs 5 at 34° C. for seedlings of flowering plants, and in many 
other cases it is lower, so that the optimum is so far removed from the fatal 
temperatures as to make it impossible that the phenomenon should be 
wholly an illusion of experimentation. Were it possible to make critical 
sets of growth-readings fairly close to the hypothetical initial values, the 
position of the optimum should be found to be higher than after long 
preliminaries, but it could hardly be pushed upwards to such an extent 
as to become uncertain by reason of its nearness to fatal temperatures. 
1 Cf. Pfeffer, Physiology of Plants, vol. ii, p. 77. 2 Ber. d. deut. Bot. Ges., 1890. 
8 Bot. Centralb., Bd. 47. 4 Annals of Botany, vol. ix, 1895. 5 Pfeffer, 1. c. 
