60 
conclude from the third column whether the numbers increase or 
decrease, they are not sufficiently certain. If we, however, arrange 
the first and the second column according to quantities of energy 
the figures rise, hence, we might expect much higher figures in the 
third column. Probably we are here beyond the maximum of growth 
retardation. On the other hand the figures rise again on prolonged 
continued illumination; this completely agrees with the fact, that 
illuminations of longer than 25 mins. always give positive curvatures. 
Now since it is known that phototropie curvature can already be 
detected after 25 mins. by means of a microscope’), a sufficient 
difference in growth between the two sides must have occurred in 
this time. If the photo-growth reaction is therefore to be regarded 
as the basis of phototropie curvature, the numbers, giving the retar- 
dation of growth to 30 minutes after the beginning of exposure, 
must provide a clue’). The occurrence of curvatures at the extreme 
apex within 25—30 mins. can therefore be sufficiently explained by 
growth retardation alone. There are however further points, which 
indicate that the acceleration of growth, following the retardation, 
has nothing to do with the establishment of the curvature. 
When the acceleration of growth is over, the growth curve still 
shows pronounced rises and falls. These are, however, not to be 
considered as oscillations about a condition of equilibrium, like those 
of a pendulum coming to rest, but must certainly be regarded as the 
reactions of zones situated below. Just as in a phototropic curvature 
the reaction first becomes visible at the extreme apex, the first period 
of retardation of growth must also represent the reaction of the 
extreme apex; the photo-growth reaction of the inferior zones will 
not occur until later. This is therefore quite comparable to the 
progress of the phototropic curvature from the apex to the regions 
further below. The magnitude of the later retardations of growth 
cannot however be deduced from the tables, since the average rate 
of growth does not remain constant, which is partly attributable to 
the great period, which itself is moreover influenced by light. *) 
1) E. Prinesaem. Studien zur heliotropischen Stimmung und Präsentationszeit. 
Zweite Mitteilung. Colin’s Beiträge zur Biologie der Pflanzen. Bd. IX. 1909. 
W. H. Arisz. Onderzoekingen over Fototropie, Diss. Utrecht. 1914; Rec. trav. 
bot. Néerlandais. Vol. XII. 1915. 
4) I neglect here the negative curvatures, which after a time may succeed to 
positive ones; these can be explained in a quite different manner, as | shall show 
in my detailed paper. 
35) H. Srerp. Ein Beitrag zur Kenntnis des Einflusses des Lichts auf das Wachstum 
der Koleoptile von Avena sativa. Zeitschr. f. Botanik. 1918. 
