fi 
M.C.S. there also lies a field of no, or doubtful negative reaction 
at 1—2 million M.C.S. Lower down we shall endeavour to get a 
somewhat better insight into these facts, but for this it is necessary 
to study the photogrowth reaction, which after BLaauw’s research *) 
constitutes the basis upon which an interpretation of the phototropic 
phenomena has to be built up. Then the curvature is to be con- 
ceived as the resultant of the effects which the umilateral illumina- 
tion has on the growth of the anterior and the posterior side. 
Beforehand | will communicate some other results of the photo- 
tropic examination. 
First of all ‘that with an illumination of 30000—45000 M.C.S., 
at which the reaction is strongest, almost hoop-shaped curvatures 
appear after 3 or 4 hours whether on the klinostat or not. Similar 
curvatures were observed by Braauw ®) in Phycomyces, still more 
similar ones by Arisz in Avena (lc. Pl. I fig. 4). It would seem 
that in that case all the parts of the stem, which, as appeared above 
are all growing, although in various degrees, partake in the curving, 
evoked by the phototrcpic stimulus. After 16 hrs these curvatures 
have fairly disappeared also in the klinostat. This is the consequence 
of the autotropism, which vaN DE SanpE BAKHUYZEN *) looks upon as 
the consequence of the obscuration, which renders the process of 
growtb-retardation, caused by the illumination reversible. 
When a part of the stem is darkened by reed covered with tin- 
foil, the curvature is much iess marked and oceurs only in the 
lighted part. It proved impossible to illuminate the sporogouia 
alone; the tip of the stem was always lighted along with the 
other parts and displayed curvatures. When the sporogonium was 
darkened and the stem alone was lighted, a curvature appeared 
all the same. It follows then that when the sporogonium has been 
amputated, the stem does not react or reacts slightly phototropically, 
this is a consequence of the traumatic stimulus; there is no question 
here about conduction of the stimulus, which makes matters much 
simpler than in the case of Avena. 
Here also the phototropically active part of the spectrum are the 
blue rays; when a glass dish filled with an ammoniac cupric oxide 
solution was placed as a screen before the light source, the effect 
was not altered appreciably, the experimental reaction time was 
about the same as when the dish is filled with water‘). 
1) Buaauw A. H., Licht und Wachstum I. Zeitschr. f. Botanik 1914. 
4) Die Perzeption des Lichtes p. 345. 
83) H. L. v. D. SANDE BAKHUYZEN, Analyse der fototropische stemmings- 
verschijnselen. Diss. Utrecht 1920. 
4) The minimal influence of the red rays has been shown above. 
