ON PLANT GROWTH HORMONES 609 
metal boxes, and in bakelite boxes, and also kept in these containers during 
the twenty-four hours of the actual experimental day. We, of course, took 
care to have the same temperature and humidity inside the boxes as in our 
laboratory. 
Now whilst the action of the auxin on the ‘ cage seedlings ’ and ‘ bakelite 
seedlings ’ was pretty much the same as that on the control seedlings out- 
side, the seedlings in the metal boxes gave us quite a different curye of 
potency ; this curve is on the average higher and the percentage variations 
aremuchsmaller. The difference is all the more striking if we bear in mind 
that the seedlings have to be removed from the boxes during a short interval 
each time they are manipulated in carrying out the test. 
What conclusions can be drawn from these experiments ? The essential 
difference between metal and bakelite boxes is doubtless that in the former, 
that is inside a conductor, an electrical field is abolished and atmospheric ions 
are eliminated. ‘The electrical field is also abolished inside the cage, but 
the latter does not completely eliminate atmospheric ions. No difference 
could be observed between zinc walls of 0-8 mm. and leaden ones of 5 cm. 
in thickness. The leaden box was suggested by our physical advisers with 
a view to the detection of a possible influence of cosmic rays. 
It was of course our aim to influence the susceptibility of the seedlings to 
auxin solutions at will, by physical means. We were as yet unable to do 
this by definite electrical fields and a supply of atmospheric ions, but we 
succeeded by means of an experimental arrangement suggested by a con- 
versation with Prof. Pohl of G6ttingen. On the supposition that the 
observed variations were caused by very weak electrical currents in the 
seedlings, we have artificially produced a potential difference in them: 
For this purpose (Fig. 14) a moist silk thread was fastened to the agar block 
and then, for instance, connected to the positive pole of the source of current, 
while the plant trough was joined to the negative pole, or conversely. We 
had for example a potential difference of 80 millivolts per cm. and a current 
of 0-0008 milliamperes. What was the effect ? 
The auxins are acids. When the silk thread was connected with the 
negative pole, the transport in a basal direction of the physiologically active 
auxin anion is accelerated. In this case we could increase the potency of 
the standard solutions—that is to say the susceptibility of the plants—to 
120,000 million A.U. per gram. On commutation the transport in a basal 
direction is inhibited and the susceptibility can be lowered to 10,000 million 
A.U. per gram. Finally, we may point out that atmospheric conductibility 
in a closed space is also known to be subject to diurnal variations. We may 
therefore safely conclude that the normal variations of susceptibility are 
also due to changes in the electrical conditions of the air. 
It will be the task of the botanists to deduce from these experiments 
conclusions concerning the finer mechanism of vegetable tropisms. But 
we think that our experiments may also be of interest to medical investi- 
gators, especially since the effect of unknown climatic influences on disease 
and the unequal distribution of births and deaths during a period of 24 hours 
has of late been the subject of renewed discussion. The physician, however, 
will have a more difficult task than ourselves, for man is a less suitable ‘ ex- 
perimental object ’ than are our seedlings. 
Finally, I should like to express my gratitude for the kind invitation to 
address the British Association at Leicester on the subject of the auxins. 
It has given me great pleasure to accept this invitation, since it offered to 
me a welcome opportunity of making the acquaintance of British colleagues. 
