174 



answer to it can be given yet, but nevertheless we can endeavour 

 to arrive a step nearer at its solution. 



We shall confine ourselves now in the first place to the stimulus 

 of gravitation. 



We have thus supposed that the static apparatus of the statocyst 

 lies' in such a position that the middle-field, which forms its most 

 sensitive part, adjoins the lower wall of the cell when the organ 

 is in equilibrium, whatever this position may be. When this position 

 be changed, if, for instance, a root be placed horizontally, the slarch- 

 grains which shift under the influence of giavitation, come into 

 contact with the less sensitive border of the apparatus; if then, after 

 some time, the tip bends downwaid, the starch-grains, again shitting, 

 will gradually come into contact with the more and more sensitive 

 parts of the apparatus till, when the tip stands verlical, they will 

 have reached the most excitable |)lace again; thus we see (hat the 

 curving downward is accompanied by a continual increase of the 

 stimulus and that the speed of this increase will t)e greatest when 

 bending takes place in a vertical plane. 



Could it be that this increase of the stimulus is the indirect 

 cause of the bending and at the same time of the choice of the 

 shortest way ? 



Of itself this "striving after the maximal stimulation", as we 

 might term it, cannot be regarded in the plant as the direct cause 

 of any movement, although it might later on be of aid in explaining 

 it ; nevertheless cases are known in which there exists a rather 

 direct connection between this striving after an ever stronger stimulus 

 and the movements. 



So, for instance, in positive chemotaxis: if e.g. spermatozoides of 

 ferns be placed in a weak solution of malic acid in which the con- 

 centration is unequal at different places, they will move towards 

 the place of the strongest concentration, i.e. in the direction of the 

 increasing concentration or stimulation. 



It is known, regarding some of the senses of man and animals, 

 sucii as the eye, the ear, and perhaps also of the static organ when 

 the organism is at rest, that they adjust themselves automatically 

 (reflectorily) to a stronger stimulus, i. e. that the same stimulus 

 which causes the sense-perception also excites other nerves and 

 through them certain muscles, which last thereby move the sense- 

 organ in such a way that it receives then the strongest possible impres- 

 sion ; thus here too we have the case of a movement with the aim 

 of increasing the stimulation. If such a comparison with the plants 

 were entirely justified, which could not be decided at present, we 



