236 BOTANICAL GAZETTE [SEPTEMBER 
Heliotropic tone.—PRINGSHEIM discusses in a long and rather technical paper 
the influence of illumination upon heliotropic tone.? By heliotropic tone he means 
that internal condition of responsiveness which determines the position of the car- 
dinal points of reaction—the liminal and optimal illumination both for positive and 
negative response, and the indifferent zone. He shows that the reaction time of 
heliotropic plants diminishes with increasing intensity of light, rapidly at first, then 
more slowly and finally becomes constant. Beyond this a false optimum (really 
temporary indifference) is reached, but only in plants grown in the dark. If such 
plants, however, after being taken from the dark are rotated for a time in a light 
to which they will later react, the indifference disappears and the reaction, con- 
trary to the earlier statements, is actually accelerated. This reaction, by a plant 
attuned to a certain light, is indeed the speediest possible at that intensity. In 
fact during the first part of the illumination of a plant of low heliotropic tone, 
the direction of the light is of no significance; for whether rotated or even illumi- | 
nated from the opposite direction, it reacts just as quickly as though continuously 
illuminated from one side. e same is true in plants of high tone with stimuli 
of low intensity. During this first period the plant is merely adjusting its tone 
to the illumination. This alteration of tone is to be considered as an effect upon 
the excitable structure itself, produced either by a like or an unlike stimulus. 
One must distinguish between accommodation or adjustment to a given illumina- 
tion and Umschaltung which determines whether the reaction is to be positive, 
negative, or none. This Umschaltung is dependent on the difference between the 
existing tone and that corresponding to the intensity of the illumination. If bo low- 
toned plant be brightly lighted it reacts negatively; if weakly, the response 1s Post 
tive. Ifa high-toned plant is brightly illuminated, it reacts positively; with weak: 
light, it does not respond at all until the tone has fallen far enough, when 4 
positive reaction occurs. The tone in both cases follows the intensity of the illu- 
mination, but rises more quickly than it falls. All hypotheses which predicate 
heliotropic tone as a constant are faulty. The phenomena line up with those at 
ready known in certain other organisms and in the human retina, whence it — 
ec that they are part of a general physiological law as to light perception 
Geotropism and heliotropism.—The mutual effect of geotropic and helio- 
tropic stimulation has been the subject of several papers, notably those by WIES- 
NER, NoLt, and CzapeK. Von GuTreNBERG, working in PFEFFER’S laboratory, 
has lately attacked the problem whether or not when they operate simultaneously 
on parallelotropic organs an alteration of geotropic tone occurs.’? He concludes, 
contrary to other interpretations, that it does not, finding it possible by choosing 
° PRINGSHEIM, Ernst, JR., Einfluss der Beleuchtung auf die heliotropische 
Stimmung. Beitr. Biol. PA. 9: 263-306. 1907. 
*° VON GUTTENBERG, H. RITTER VON, Ueber das Zusammenwirken vor pag? 
pismus und Heliotropismus in parallelotropen Pflanzenteilen. Jahrb. Wee 
45:193-231. 1907. : 
eae So eT 
