GEOTROPISM. I 



439 



Experiments on this subject are as yet scanty, so that we need not attempt to 

 arrive at any definite decision between the two theories. [Meanwhile Fitting's 

 (1905) careful investigations have made us more thoroughly acquainted with 

 these geotropic phenomena. This author has studied in detail the results 

 of intermittent stimulation, and was able to show that the duration of a 

 single stimulus may be shortened at will, and that by summation of these 

 stimuli a movement is finally induced. Fitting has also studied the relation 

 between the period of duration of the stimulus and of the interval of non-stimulus 

 in the case of intermittent stimuli, and has found that geotropic curvature 

 invariably takes place if the intervals are ten times as long as the periods 

 during which the stimulus is applied. He has also shown quite clearly that only 

 the bending and not the perception of the stimulus is impossible on the klinostat. J 

 In addition to the dttration of the geotropic stimulus we must also take 

 into consideration its intensity. The variations in the amount of the stimulus, 

 however, as observed in different regions of the earth, are so minute that 

 it is quite impossible to deal with them experimentally, even if they were 

 more accessible to the observer than they really are. Knight's discovery 

 relieves us from all difficulty in this respect, for we may increase the centri- 

 fugal force to any extent we please, and we may thus study the dependence 

 of the latent period on the amount of this force. If we vary the centrifugal 

 force so that on the one hand it amounts to thirty-eight times the value of g. 

 (gravity) and on the other hand reduce it down to 0-0005 g- the reaction takes 

 place (in the root of Vicia faba, Czapek, 1895) in the following times : — 



;h. ih. lih. i^h. ■2\h. 3I1. 4h. 5h. 6h. 81i. 



38-35 §:• 28-10 7-43 3-5 09 06 0-5-0-4 0-2-02 0003 0001 00005 



From these experiments it follows first of all that the plant responds in the 

 same way but more slowly to a pulling force a thousand times less than g. 

 As in all movements manifested in response to a stimulus so in the case of 

 geotropism the stimulus must reach a certain amount, the so-called 'liminal 

 value", before a reaction follows. The reaction follows all the more rapidly the 

 more vigorous the releasing force is, and we may, therefore, conclude that the 

 effect of the stimulus or the excitation in the plant is so much the greater the more 

 vigorous the centrifugal force is. No investigations have as yet been made on 

 the effect of still greater intensities of centrifugal force. It must not be assumed 

 that the excitation increases pari passu with the increase in the centrifugal force, 

 because this force will in the long nm have an injurious effect on the plant, or 

 at least retards growth (Andrews, 1902). It may also be possible to determine 

 experimentally an apex on the stimulus curve (region of greatest excitation) and 

 an upper hmit of stimulus in addition to the already known liminal intensity. 

 We may alter not only the duration and intensity but also the incidence of the 

 geotropicstimulus. If a shoot be so placed that it grows in a straight line upwards, 

 that is to say, parallel, but in the opposite direction, to that of the geotropic 

 stimulus, there is no reaction at all, or, to be more accurate, there is no geotropic 

 curvature. If, however, the shoot be placed at an inclination to the vertical so 

 that the line of direction of gravitymakes an angle with the axis, acurvature takes 

 place, owing to the fact that on the under side growth is accelerated and on the 

 upper side retarded. The influence of gravity will have all the greater (but purely 

 mechanical) effect the more nearly the stem approaches the horizontal. In that 

 position gravity should have its maximum effect, and if we go on turning it over, 

 that effect will again be diminished, until finally, in the inverted position, it will 

 have reached zero. Recent researches do not, however, confirm this view. 

 Czapek (1895), employing various methods of producing the excitation, found 

 that the maximum effect was produced when the angle 135° downwards was 

 reached. Roots behave exactly in the reverse way, responding most when placed 



