4 
Phototropism (Heliotropism): As appears from observations in nature, 
the growing sporogonium-stems are very sensitive to phototropic and 
geotropie stimuli. Illuminated by unilateral clear daylight the long- 
stems curve distinctly already after + 10 minutes, in the sunlight 
after 5 minutes. 
The following observations were made in the dark room of the 
Botanical Laboratory of Utrecht *), where gas is absent; “laboratory 
air’ caused no trouble; the objects, it is true, displayed many 
nutations, but this was quite the same in the open air. The 
temperature was 13°—15° C.; the observations were made with red 
light (electric lamp with safranin-bell). A preliminary test, in which 
the light was acting for hours at a distance of 1 or 2 d.m., proved 
that only a very slight curvature was brought about. This curvature 
was, moreover perhaps partly thermotropic. 
The material, almost a pure culture, taken from the shady side 
of a ditch near Amersfoort, was cut into pieces with a sharp knife. 
These pieces just filled the ordinary boxes generally used for 
Avena. Subsequently these boxes were kept in the dark for a week 
at the very least, before they were used for the experiments. Only 
those objects which had stems of 15-—-25 mm. were used for the 
experiment, the larger ones were too feeble. Many of the sporogonia 
had to be removed before the experiment began, because they were 
crooked or displayed nutations; only the vertical stems were used, 
20 of them serving as objects in every experiment ®). 
The curvatures were observed macroscopically ; the black sporo- 
gonium indicated the deviation distinctly. In order to preclude any 
influence of chance nutations on the result, the time after which 
10 out of the 20 objects displayed the curvature was considered as 
the experimental reaction time *). Unless the contrary appears from 
the experiment itself, the material was placed on the klinostat after 
the excitation (rotation time + 6 minutes). 
The candle-power of the various electric light-sources was measured 
with WeBer’s photometer; some oscillations may have been caused 
by a modification of the net-tension. These, however, did not affect 
the principle of our research. 
1 Owing to the kindness of Prof. WeEnT this research could be continued during 
the holidays, which removed the difficulty that the growth of all the stems is 
completed in a few weeks. 
*) Material grown specially for the experiment, would have yielded better results 
no doubt. This is in course of preparation. 
5 W. H. Artsz (Onderzoekingen over fototropie. Diss. Utrecht 1914) considers 
the ‘‘experimental reaction time” to be the time required for a curvature just 
noticeable to the unaided eye. 
