SUPPLEMENT 133 



a certain time, and which is accentuated by the next application of the stimulus. 

 This view is supported by the results obtained by FITTING in intermittent 

 stimulation. If the duration of the stimulation be first of all equal to that of 

 the alternate rest period, stimulation, in order that curvature may follow, 

 must be continued until the sum of the individual stimuli is equivalent to the 

 presentation time. If the rest periods be increased until they are five times 

 as long as the stimulation periods, the result follows, and the reaction still 

 occurs after the same time of stimulation as it would if the stimulation had 

 been continuous. Hence the absolute duration of the individual stimulation 

 is of no significance. The rest periods between the individual stimuli cannot, 

 however, be lengthened indefinitely, for the effect of the individual stimulus 

 gradually wears off, and this takes place when the rest period is twice as 

 long as the stimulation period. The effect of lengthening the rest periods, 

 however, makes itself felt in an increasing of the presentation period: e.g. 

 12-15 minutes, as opposed to 6-7 normal time, when the presentation period 

 is to the rest period as i : u. It is obvious that lengthening of the rest periods 

 also delays the commencement of curvature, i.e. the latent period is lengthened. 

 The ' change ' which a single stimulus induces in the plant is termed the ' ex- 

 citation ', and this excitation must reach a certain intensity before any visible 

 reaction takes place ; what excitation consists in, however, is unknown. 



439, 1. 20, for latent period read latent and presentation periods 



11. 20-39, f or H we vary . . . liminal intensity read When the centrifugal 

 force is less than gravity the latent period is lengthened ; if, as in Vicia Faba, 

 the latent period amounts to one and a half hours when the force = i g., it rises 

 to four and a half hours when the force = 0-014 g. (BACH, 1907). This figure 

 is far from expressing the lower limits of the centrifugal forces which produce 

 a geotropic effect. CZAPEK (1895) observed geotropic curvature taking place 

 after 8 hours with a force = 0-0005 g- '> BACH, in opposition to CZAPEK, found 

 that there was no shortening of the latent period when the centrifugal force 

 was raised from I to 100 ; on the contrary, he found that the dependence of 

 the presentation period on the intensity of the stimulus was quite different 

 in nature from that of the latent period, as is shown by the following table : 



Centrifugal force in g. . . .27 19 10 6 3 2 i 07 0-6 0-4 0-15 

 Presentation period in minutes . 0-25 05 i 2 3 4 8 10 25 30 50 



It is possible also to shorten very considerably the presentation period, 

 more especially by employing high centrifugal forces probably much more 

 than is indicated in the above table. From a practical point of view it is not 

 possible, however, to apply high centrifugal force suddenly and permit it to 

 act only for a few seconds, otherwise one should certainly find that stimuli 

 of a few seconds' duration should suffice to induce a geotropic movement. There 

 can be no doubt that more vigorous stimulation brings about more vigorous 

 excitation, but why a more vigorous excitation does not lead to a shortening 

 of the latent period cannot be investigated here (comp. FITTING, 1905). 



1. 51 P. 440, 1. 6, for Recent researches . . . shoots upwards, read Recent 

 researches (FITTING, 1905) have indeed established this view. The point may 

 be most clearly shown by stimulating intermittently two opposite sides of 

 a plant, when it is seen that whenever the two positions of stimulation show 

 the same deviations from the horizontal, curvature does not exhibit itself, but 

 that a reaction follows if the angles with the horizontal be unequal. The 

 curvature which takes place, indeed, is just that which is aimed at when the lie 

 of the plant is nearest to the horizontal, and which overrides the contrary 

 curvature in the opposite lie. It is possible also to show both for the root and 

 the shoot that the inverse, as well as the normal, lie is a rest position, although 



