ON PLANT GROWTH HORMONES 605 
lower than auxin ; according to its very probable formula C,,H )O, it is 
isomeric with auxin-lactone, from which it differs however completely, 
already by its acid nature. A close relationship to auxin must, however, be 
assumed on account of various chemical properties; the physiological 
activity is of the same order of magnitude as that of auxin and its lactone 
round about 50,000 millions A.U. per gram. Within the last few months 
we have prepared 120 mg. of the new crystalline substance which we will 
designate as aquxin-b ; the substance first isolated from urine will hence- 
forth be called auxin-a. Of the four oxygen atoms of auxin-b, two belong 
to a carboxyl and one to a hydroxyl group. The course of the mutarotation 
once more indicates that this hydroxy] is in a §-position with reference to the 
carboxyl. The fourth oxygen is present as a ketogroup: auxin-b yields 
a crystalline semicarbozone and on treatment with methyl alcoholic hydrogen 
chloride forms a crystalline lactone of the dimethyl acetal. Finally, we can 
also give some indication of the position of the carbonyl group relative to 
the carboxyl. At its melting point auxin-b rapidly evolves carbon dioxide 
and passes into a neutral substance. As far as we can see this is only com- 
patible with the assumption that auxin-b is a B-ketonic acid (Fig. 8) ; we are 
surprised that the substance can nevertheless be isolated. We have not 
Auxin-a G20; 
C rprkt tri-of- a) 
R-Ci 1 Ct -C TCE, OHO OK 
AAuxin-6 Ce tbo 
(auxene-ofon-acid) 
R-CH-CH-CO- Co, 
Fic. 8. 
yet succeeded in transforming one of the auxins into the other, but we have 
been able to oxidise auxin-b with permanganate to the same dicarboxylic 
acid with 13 carbon atoms which we had already obtained from auxin-a ; 
the double bond must therefore be in the same position in both molecules. 
Finally, it should be mentioned that the crystals of both auxin-a and of 
auxin-b completely lose their physiological action on keeping for some 
months. All these facts make it certain that our two auxins are very 
closely related. 
The occurrence of auxin-a in urine brought this vegetable hormone 
(‘phytohormone ’) also into the realm of animal physiology. We have 
given considerable attention to the problems arising in this connection, but 
I can only mention the results here very briefly. Adults excrete about 
2 mg. of auxin-a per day, independently of age or sex.. Urine excreted a 
few hours after a meal has the highest auxin content (Fig. 9). During a day 
of fasting less auxin is eliminated and the characteristic ‘ auxin peak’ does 
not appear. We have tested various diets and found that after ingestion of 
glucose, starch, or egg white no auxin peak appears, but that such a peak 
does appear after ingestion of salad oil (arachis oil; Fig. 10), and butter 
(Fig. 11). Fats and oils contain the auxins in a free or in an esterified form. 
A hydrogenated coco-fat, in which the auxins had been modified by reduc- 
tion and inactivated as regards plants, produces no auxin peak (Fig. rr). 
The auxin peaks do not therefore arise indirectly after ingestion of fats. 
A large part of the eliminated auxin is derived from the fats of the food. 
