ON PLANT GROWTH HORMONES 607 
‘ by-product’ for us chemists, but are of great biological interest. The 
potency of 50,000 million A.U. per gram of our phytohormones is only an 
average value ; the actual potency varies from day to day, and in the course 
of time we have observed with standard solutions all degrees of potency 
between about 10,000 million and 100,000 million A.U. per gram. Our 
suspicion has been more and more strengthened that these large variations 
are not due to experimental error but to unknown external causes, which 
can even exert their influence in our dark laboratory kept at constant tempera- 
ture and humidity. We paid special attention to the various atmospheric 
conditions, but no certain relationship could be deduced even from observa- 
tion extending over several months. My colleague, Prof. Went, informs 
me that the possibility of such unknown influences of the weather has often 
been canvassed in vegetable physiology, but that all experiments aiming at 
Denendence of auxin elimination 
upon the nature Sf wegested pe a 
age oe Gutter Aueragenated 
OTE i 703° H ak (7009); 
the discovery of definite relationships, e.g. to ‘ atmospheric electricity,’ have 
failed. 
We did not make any progress until we undertook the examination of the 
potency of the auxins at hourly intervals during periods of 24 hours. This 
examination was more easily planned than carried out, but the skill and 
perseverance of my collaborator, Dr. Haagen-Smit, overcame all technical 
difficulties. Whilst we normally carry out tests on 300 to 400 seedlings per 
day, up to 1,500 seedlings had to be examined on the following important 
experimental days. Since the age of the seedlings in the test reaction is not 
a matter of indifference, we use them exactly 88 hours after sowing. ‘The 
determinations of potency during the 24 hours of an experimental day were 
therefore only valid, when the sowing had likewise taken place at hourly 
intervals, four days previously. The results of the experiments are repre- 
sented in Figs. 12 and 13, in chronological sequence. I would here sum- 
marise them by a few empirical rules : 
We found that in the morning hours—not always but mostly—there occurs 
a pronounced maximum of potency. Thus, for instance, one and the same 
