356 BOTANICAL GAZETTE [aay 
positive response of an organism to a stimulus is as follows: the 
organism, swimming about at random and coming to that portion of 
the medium in which the stimulant is present, has its movements 
directly modified by the stimulating agent, so that the direction of 
its axis is turned toward the region of stimulation; it then proceeds 
in a straight line toward the stimulus. This is the “local action theory 
of tropisms”’ discussed by JENNINGS (18, 19), and includes the “stro- 
phic” movements of RoTHERT (39) and the “topotactic’’? movements 
of PreFFER (38). According to the other view, the organism, 
coming into the region of a stimulant to which it reacts “positively,” 
enters this region without reacting, but upon tending to leave the 
region, it reacts by turning back; the “positive” response is thus 
obtained by a series of “negative” reactions whenever the organism 
tends to go in a direction leading away from the source of stimulation. 
This is the reaction described by JENNINGS (18, 19) for many pro- 
tozoa and bacteria, and includes the “apobatic’’ movements of 
ROTHERT, and the “phobotactic’”? movements of PFEFFER. Similar 
explanations to those accounting for the positive reactions in the 
two views are given for negative reactions. 
According to the former account, the source of stimulation is the 
unequal distribution of the stimulant on different parts of the organ- 
ism; this acts locally on the organism, directly influencing the motor 
organs; and the essential part of the reaction is the orientation of 
the organism so that its axis is placed parallel to the direction of 
greatest stimulation. According to the latter account, the se tiie 
of stimulation is the change of the conditions to which the organism 
is subjected; this change acts on the organism as a whole; and the 
essential nature of the reaction is that the organism shows no response 
in passing toward the optimum, but reacts by turning back upon 
tending to pass from the optimum. 
The reaction of motile plant cells to chemical stimuli has been 
studied by PFEFFER (36, 37) in ferns, Marsilia, Selaginella, mosses, 
liverworts, Chara, bacteria, the swarmspores of Saprolegnia, and 
certain flagellates; by Liprorss (24, 25) in Marchantia and 
Equisetum; by Snrpata (43, 44) in Salvinia and Isoetes; by 
VoEGLER (51) and BuLier (3) in ferns; and by ROTHERT (39); 
KNIeP (22), and JENNINGS and CrosBy (20) in bacteria. 
