ON PLANT GROWTH HORMONES 6or 
growth effect is called one Avena?! unit (A.U.). We may further mention 
that phototropic and geotropic curvatures are also caused by auxins ; thus 
the shaded side of the shoot (in the former case) or the under side (in the 
latter case) are found to have a higher auxin content than the opposite side. 
The growth test of Went, by the help of which we were able to isolate the 
auxins, is best shown ina film. (Demonstration.) 
When some two and a half years ago we were searching for a suitable 
source from which to isolate auxins, our attention was directed to the high 
auxin content of human urine. We know now that the isolation of the 
auxin from urine implies a concentration of 21,000 times ; its isolation from 
the vegetable sources which were known at this time would have meant 
a concentration of at least 500,000 times. Under these circumstances 
it will be readily understood that we followed the line of least resistance and 
first attempted the isolation from urine, although this is an animal source, 
a circumstance which many botanists no doubt felt to be an esthetic defect 
in our work. 
We have earlier described the isolation of auxin from urine. The quan- 
tity of crystallised auxins which we have obtained so far from urine—about 
400 mg.—would have just been sufficient for three or four com- 
bustions before Pregl introduced micro-analysis seventeen years ago. If, 
now, we had devoted all our energy to the preparation of pure auxin, it 
might have been possible to convert the trail or the footpath which led us 
to the crystalline substance into a high road. For the problem as a whole 
it seemed to us, however, more important to devote a part of our time to 
the study of new physiological problems which presented themselves ; this 
also helped the chemical investigation in many ways ; we were, moreover, 
able to compensate to some extent for the shortage of material by improve- 
ments in the preparative micro technique. 
Micro-analyses, determinations of the molecular weight, and titrations, 
led us to the formula C,,H;,0; for auxin; this composition also agrees 
with that of the derivatives obtained so far. In addition to a carboxyl 
group the molecule contains three alcoholic hydroxyl groups ; the course 
of the hydrogenation shows that auxin contains one double bond and one 
carbon ring ; it is therefore a monocyclic trihydroxy carboxylic acid with 
one ethenoid link. If we call the basic hydrocarbon C,,H3, auxane, then 
the growth substance is auxene-triol-acid. 
A second crystalline substance of equal physiological potency was recog- 
nised in auxin lactone C;,H3,O,. Like auxin the lactone exhibits mutarota- 
tion, which is evidently due to the attainment of an equilibrium between 
the acid and its lactone. A constant rotation is reached after two or three 
hours, and if we may utilise the results of Haworth and his collaborators by 
way of comparison, we can deduce the size of the lactone ring. According 
to these authors 8-lactones usually attain equilibrium in a few hours, whilst 
y-lactones require days. We consider it therefore probable that in our 
case the substance is a 8-lactone. If there were hydroxyl groups in both 
the y- and 5-positions, the formation of a y-lactone would very probably be 
favoured, whence we conclude that there is in auxin no hydroxyl group in 
the y-position (with respect to the carboxyl group). 
So far we have only been able to sacrifice 126 mg. of the substance for 
degradation experiments. Miss Erxleben has succeeded in isolating two 
important oxidation products. The first oxidative attack was directed 
against the double bond. On treatment of 25 mg. with permanganate in 
sodium carbonate solution a crystalline optically active acid was obtained. 
1 Avena sativa = oats. 
