1279 
thereupon the light curvature begins to approach its maximum, 
there remain only traces of the other. There can in this case be 
no question of a clear reinforcement of the first reaction by the 
second. On the other hand, this reinforcement may be expected to 
be very pronounced, when so much time elapses between the stimuli 
that both curvatures reach their maxima at the same time. 
It is convincingly proved by experiments which are described 
in the following section, that a complete summation does indeed 
take place in this last case. Moreover the surmise was confirmed 
that, when -a small light-stimulus and a geotropic induction are 
applied simultaneously or in rapid succession, the geotropie 
curvature reaches its maximum before the phototropic one is clearly 
visible and has more or less disappeared again, when the latter has 
attained its greatest value. Mrs. RurreN’s results thus find a simple 
explanation. 
We must, however, guard against concluding from these data 
that there is no modification of the phototropic reaction, due to 
gravity, or of the geotropic curvature under the influence of light. 
Prerrer ') has already pointed out the possibility of such changes. 
Excepting, however, certain special cases relating to plagiotropic 
organs (e.g. rhizomes of Adoxa, in which case Stan. first showed 
a change in the geotropic reaction under the influence of light) this 
phenomenon has not been completely demonstrated. (Compare Gurrex- 
BPRG*)). I have succeeded in finding several’ examples of this in 
Avena seedlings. The reversal of the geotropie reaction after omni- 
lateral illumination of certain duration may be put forward as a 
striking case. Details and discussion relating to these phenomena are 
embodied in sections 3 and 4. 
§ 2. Summation of phototropic and geotropic curvatures. 
After it had been found, in a number of preliminary experitnents 
of which details may here be omitted, in what time light- and 
gravitational curvatures, as reactions to stimuli of definite strength, 
reached their maxima, I arranged the experiments in the following way. 
I used for illumination a 10 candle-power Osram-lamp fed by an 
storage battery, which I kept constant at 10 volts. Since this had 
however drawbacks, the battery was strengthened later and the 
current was kept at the desired strength by the use of an adjustable 
') W. Prerrer. Planzenphysiologie. 1 Aufl. 1881, Bd. 2, p. 338. 
*) H. Rrrrer von Gurrenperc. Ueber das Zusaramenwirken von Geotropismus 
und Heliotropismus in parallelotropen Pflanzenteilen. Pringsheim’s Jhrb. XLY. 1908, 
‘ 
