9 
sometimes some minutes later. The mean retardation of 6 observations 
amounted to 35°/, (min. 25°/,, max. 47°/,); the rate of growth 
seems to rise very slowly to the mean value before the illumination. 
Whether it rises beyond this value later on, was difficult to ascertain, 
since owing to the nutations the observations could be continued 
for an bour only. 
1stobject = 12"—12h10™dark growth 85u 24 object dark ZO u 
12h 10™—12h 20" i han Rel 
12" 20m—14 25 25” light x, lieht} 
12h 5m 120 30n ada Sne 
12h 30™—12 40™ dark 64u © Det 
12h 40m™—12h 50m 74u ae oe 
13h 50m™—1h % 19 u sean OON 
With unilateral illumination the anterior side of the still growing 
green parts is illuminated more intensely than the posterior side, 
which is due to absorption into the tissue. This was made out by 
photographs taken with sensitive paper of the silhouette’). The: 
stronger retardation of growth at the anterior side, to which the 
unilateral illumination has assigned more energy, will therefore 
engender a positive curvature, in other words: the case of Helianthus 
hypocotylidons *). Only in Pellia the sensitivity is not so great and 
as yet no rise in the rate of growth beyond the normal has been 
detected during the oscillations, which may be called forth by the 
illumination *). To consider the first growth-retardation caused by 
the illumination as not belonging to the photogrowth-reaction (as 
Srerp *) does in Avena) makes no sense here, where conduction of 
stimulus is out of the question and the curvature may appear much 
quicker. 
The results of the phototropic observations: the initial slow rise 
of the positive reaction up to + 45000 M.C.S., the succeeding slow 
fall down to 1 or 2 million M.C.S., with a long stage of indifference 
without distinct negative reactions, where anterior and posterior side 
react alike, all this points, in my opinion, to a very gradual course 
of the growth-retardation curves. Probably the latter will show a 
1) Absorption is insignificant in the fully developed clear stems, but here also 
the curvature is slight. 
2) A. H. Buaauw. Licht und Wachtstum II. Zeitschr. f. Botanik VII 1915. 
5) That also in Pellia the photogrowth-reaction must be accelerated after the 
growth-retardation might be inferred from the autotropism, the straightening in 
the dark. 
4) H. Sierp. Zeitschr. f. Botanik XII 1921. 
