ON PLANT GROWTH HORMONES 
(AUXIN A AND AUXIN 8B) 
BY 
PROF. DR. FRITZ KOGL, Urrecut. 
(Ordered by the General Committee to be printed in extenso.) 
THE experiments which I am about to bring before you were mainly carried 
out in conjunction with Dr. Haagen-Smit and Dr. Hanni Erxleben; they 
are based on modern researches on the physiology of vegetable growth— 
researches which we owe chiefly to the school of Prof. Went of Utrecht. 
As a preliminary I would summarise the chief results of these researches as 
follows : Whilst animal growth takes place almost exclusively by multi- 
plication of cells, we must distinguish in plants between cell division and 
cell extension. ‘The obvious increase in volume which is seen in higher 
plants depends principally on cell extension. Fig. 1 shows two stages in the 
<— Growth 
substance 
ts formed 
tn the top 
of the 
Avena- 
coleontite. 
(Avena sativa 
= oats ). 
ek 
removed a 
‘deficiency con- 
dition"is induced 
resulting in an 
interruption of 
substance 
wanders 
extension 
of cefts. 
During this 
interval new 
wth can be 
Grought about 
6y See a 
btock of agar 
containing Guxins 
of the Clock of 
agar is placed 
on the cofeontife 
stump asymmetri- 
cally, a curvature 
results from the 
unequat growth 
of the two sides, 
, Under certain 
4 Conditions this 
curvature 
ts according to 
FWWent 
Lrgnortional 
to the concentration 
auxin. 
Of the angle (o:) is 10° 
the tis 
Avena-unit. 
sds 
=< 
+ Shototronism: 
vege 
side ts s found 
to have a 
higher 
auxin content 
38, 3 
-Seotronism: 
the under side is 
Jound tohave a higher 
auxin content 
extensional growth 
during ajew hours. 
of various sources. than the opposite side. 
BGs 2s Fic. 3. 
cell extension of oats seedlings. This extension takes place under the 
influence of definite growth substances, which may be termed auxins. 'The 
mode of their physiological action and recognition is represented diagram- 
matically in Figs. 2 and 3. ‘The active substances are formed in the top of 
the seedling and wander from there into the base ; if the top is removed a 
* deficiency condition ’ is induced, resulting in an interruption of extensional 
growth during afew hours. During this interval new growth can be brought 
about by auxins from various sources. If the block of agar containing 
growth substance is placed on the coleoptile stump asymmetrically, a curva- 
ture results from the unequal growth of the two sides. Under certain 
conditions this curvature is, according to F. W. Went, proportional to the 
concentration of the growth substance ; if the angle of curvature is 10° the 
