42 
Leitch . — Experiments on the Influence of 
With regard to a possible explanation of the second maxima shown in 
the time-growth curves, at 30° and 35 0 , reference may be made to a recent 
paper of Miss Sophia Eckerson (’14) on Thermotropism in Roots. The 
author shows that changes in the thermotropic reaction are due to changes 
in the permeability of the roots at certain critical temperatures. In the case 
of Pisum , a decrease in permeability takes place between 30° and 40°, result- 
ing in a negative curvature in roots exposed to a higher temperature on one 
side, in that range. It is possible that roots exposed to temperatures within 
that range might show a secondary elongation due to osmotic phenomena, 
which, superimposed on a simple time-growth curve, would give rise to 
a temporary second maximum. Too much stress should not, however, 
be laid on this coincidence. As already stated, Kuijper’s respiration deter- 
minations also show fluctuations at temperatures between 30° and 40°, and 
it is possible that both these and the second maximum in growth may 
be due to deeper, unanalysed metabolic processes. 
Summary. 
The relation of growth to temperature can be expressed as a uniform 
curve from — 3° to about 29 0 . It is not a van ’t Hoff curve, but shows a very 
close resemblance to the curves found by Krogh for ‘ standard metabolism 
in animals ’, and by Kuijper for respiration in Pisum. 
Above 29 0 the relation can no longer be expressed as a curve, but for 
each higher temperature a different curve must be constructed to express 
the rate of growth in successive time-intervals. Between 30° and 40° these 
curves are not simple time-curves, and no extrapolation is possible. 
For growth there is a well-marked optimum temperature. The general 
applicability of this term to physiological processes is considered, and 
a definition offered. A further point, the maximum-rate temperature 
is distinguished and defined. 
For growth, the minimum temperature is — 2° ; the maximum 44°5° ; 
to the degree of accuracy found possible the optimum temperature is 
between 28° and 30°, and the maximum-rate temperature is 30*3°. 
o 
This work was carried out in the University Plant-physiology Labora- 
tory in Copenhagen during my tenure of a Carnegie Research Scholarship. 
I have to express my indebtedness to Professor Johannsen for the suggestion 
to perform these experiments, and for the opportunity to do so ; and to 
Dr. Boysen- Jensen for many helpful suggestions, for his continued interest 
in the work, and for much of that sceptical criticism which keeps one in the 
strict path of verification. 
