( 1027 ) 
Up to 100 MCS. there was an increase in the magnitude of 
the curvature, at first more rapid and then somewhat lower; from 
100 M.C.S. to 400 M.C.S. the same magnitude, after which 
there is a decrease. Below 7 M.C.S. the curvature could not be 
measured by this method, up to 2 M.C.S. the curvature, as a faint 
inclination of the apex, was still clearly visible macroscopically. 
These apical inclinations have been noticed before, e.g. by Rurerrs’), 
who did not however recognise them as phototropic curvatures, 
because they also arose without previous stimulation. Control-experi- 
ment which L made, showed however that when coleoptiles, which 
showed absolutely no inclination of the apex, were placed in the 
dark and care was taken that they were previously stimulated 
neither geotropically nor mechanically by touch or similar agency, 
they showed no apical inclination, whereby, however (Ruraers 1. ¢. 
p. 56) attention was only paid to the curvature at right angles 
to the plane of nutation. 
Below 2 M.C.S. the curvature was so faint, that macroscopically 
it could not be recognised with certainty. The microscope is likewise 
inadequate for this. The smallest curvature observed was about */, mm. 
at 1.4 M.C.S. 
No limit can therefore be fixed below which no curvatures arise, 
but that there are curvatures which at present escape our obser- 
vation, is highly probable. The suggestion is obvious, and the course 
of the curve is an argument in favour, that the curve should be 
continued to point 0. The significance of this is, that every quantity 
of energy gives rise to a definite degree of curvature. 
Lach quantity of energy reacts on the plant and is expressed by 
a curvature of definite maximal strength. 
Since the phototropie curvatures with which these observations are 
concerned, were all to some extent counteracted by gravity, it was 
desirable to bave for comparison experiments, from which the uni- 
lateral action ‘of gravity bad been eliminated. For this purpose Fit- 
ting’s intermittent clinostat was used, which makes it possible to 
place a plant during equal intervals alternately in positions which 
differ from one another by 180 degrees, so that the action of gravity 
in one position balatces that in the other. In 2 minute periods no 
appreciable curvature arose in unilluminated plants after 6 hours. 
Out of every + minutes the plants were for 2 minutes in a position 
in which they could be examined and drawn under the microscope. 
1) A. A. L. Rureers, De invloed der temperatuur op den praesentatielijd bij 
geolropie, Dissertatie, Utrecht 1910, 
