ital Fe ee OG a ae 
: Fs 
‘ wees 
1673] SCHLEY—GEOTROPIC STIMULATION 481 
elongation in growing plant organs. EcKERSON (8) has shown that 
acids decrease the period of after-ripening of certain dormant seeds. 
The activity of these acids on germination is due in part to the 
transformation of zymogens into active enzymes and to favoring 
the activity of those enzymes. In both germination and growth 
it is probably partly due to increasing the water-absorbing power 
of the colloids, especially of the protoplasm. 
In view of the effect of acidity on the absorption of water and 
on the growth rate, it was thought best to reexamine the two halves 
(concave and convex) of geotropically stimulated and responding 
organs for difference in acidity, and incidentally for other features 
such as the sugar content. F ollowing FIscHER’s suggestion, it was 
thought that relative growth on the two sides might parallel the 
acidity. A later work will examine into this condition of tropic and 
nastic responses, and a later paper will relate the data more fully 
to the literature. 
Etiolated seedlings grown upright on boards under a spray 
have been used throughout the experiment. The temperature main- 
tained was approximately 16°C. When the seedlings were 6-8 cm. 
high, they were used in making tests of acidity upon either the 
unstimulated or the stimulated organ. For the latter tests the 
board containing the seedlings was turned on the side so that the 
seedlings were brought at right angles to gravity. Tests for acidity 
were made at various intervals of presentation and reaction time, 
ranging from 7 minutes to 48 hours. In making the tests on the un- 
stimulated shoots, the vertical stem was split longitudinally bed reght 
and left halves (in the plane of the cotyledons), and also into back 
and front halves (at right angles to the plane of the cotyledons) 
(figs. 1 and 2). For the tests on the geotropically stimulated seed- 
lings the shoots were split longitudinally into upper and lower halves, 
the former being the half away from the direction of the stimulus 
and the latter the half next the direction of the stimulus. a 
reaction these showed as concave and convex halves crscpeneee 
These terms hold throughout the paper (figs. 3 and 4). ‘ és 
terminal 4.5 cm. was used, since this region included net ite 
of curvature and since tests on acidity in successive ad 
from the tip downward showed the maximum acidity to be inclu 
