THE STAT0L1TH-THE0RY 599 



Mobility of the starch-grains entails a further advantage, inasmuch 

 as it ensures that the ectoplasts associated with the walls which run 

 parallel to the " direction of gravity," soon become free from statoliths, 

 and hence are preserved from the disturbing influence of the accidental 

 stimulation which would otherwise result from every slight vibration. 

 Owing to their freedom of movement, the statoliths are also withdrawn 

 from the orientating influence which the nucleus often exercises upon 

 such structures. Otherwise, namely, leucoplasts and chloroplasts, with 

 their included starch-grains, are frequently aggregated around the 

 nucleus/and rendered incapable of responding to the influence of gravity 

 by the high viscosity of the cytoplasm. In certain positions of the 

 nucleus, starch-grains " anchored" in this way could not act as statoliths. 



Before concluding this general discussion, it may once more be 

 remarked that the statolith-theory agrees very well with all the experi- 

 mental data which bear upon the subject of geotropism, or at any rate 

 that none of these data are irreconcilable with the theory. Attention 

 may be specially directed to the fact that geotropic stimulation attains 

 its maximal intensity for a parallelotropic organ, when the latter is 

 placed horizontally. 302 This result, which was assumed long ago by 

 Sachs, confirmed by the researches of F. Darwin and Miss Bateson, and 

 finally established beyond possibility of doubt by the recent work of 

 Fitting, can be deduced a priori from the statolith theory ; for it is only 

 in the horizontal position of an organ that all the statoliths will be resting 

 upon the physically lower portions of the ectoplast at the end of the 

 period of migration. If the displacement from the stable position amounts 

 either to more or to less than 90, if, in other words, the organ is 

 placed obliquely it is only a variable proportion of the statoliths that 

 come to lie upon the lowermost of the longitudinal walls of each statocyst. 

 Moreover only a limited area of the wall in question will become covered 

 with statoliths in these circumstances ; the rest of the falling starch 

 will come to rest against the upper and lower transverse walls, and 

 will be ineffective as far as stimulation is concerned. The maximum 

 intensity of stimulation therefore coincides with the horizontal position. 



After these introductory remarks, attention may be directed first to 

 the anatomical characteristics of the statolith-apparatus in roots, stems 

 and other geotropically sensitive organs, and subsequently to the 

 experimental evidence upon which the statolith theory is founded. 



B. THE STATOLITH-APPARATUS IN ROOTS. 



The positive geotropic curvature of ordinary roots always takes 

 place in the sub-apical growing zone. Perception, on the other hand, is 

 mainly located in the actual tip 303 ; it is, however, not absolutely confined 

 to this region, the growing zone being also sensitive, in all cases which 



