002 SENSORY SYSTEM 



According to the author's own observations, the growing zone of the 

 primary root is also sensitive though less so than the root-tip in Vicia 

 Fdba, Lwpinus albus and Phaseolus multijlorus, and the same condition 

 probably prevails in many other roots. In the aforesaid species, the 

 greater sensitiveness of the root-tip is reflected in the more perfect 

 development of the statolith-apparatus in the cap. In the less sensitive 

 growing zone, the perceptive capacity is confined to the periblem, which 

 contains numerous starch-grains ; in Vicia Faba these are movable in 

 the most actively growing region, but in other cases they are always 

 irregularly distributed through the cells. 



While the principal statolith-organ of the root is located in the root- 

 cap in the great majority of cases, there are some exceptions to this rule. 

 In Selaginella Martcnsii, for example, the root-cap is entirely devoid of 

 starch, but falling starch-grains occur in the inner layers of the periblem, 

 within a zone which begins at a distance of '13 to '16 mm. from the 

 growing-point and extends over *27 to "34 mm. In Trianea bogotensis, 

 again, the root-cap apparently contains neither starch-grains nor any 

 other bodies of high specific gravity; but, here also, the cells of the inner 

 periblem-layers are provided with falling starch-grains throughout the 

 region of curvature. In accordance with this fact, Nemec found, that 

 if roots of Trianea are deprived of their tips and are then immediately 

 exposed to geotropic stimulation, they exhibit appreciable curvature after 

 three hours. 



It was known to Sachs, that lateral roots of the second and higher 

 orders are very feebly geotropic, or even quite indifferent to the influence 

 of gravity. The author himself has found that the statolith-apparatus 

 of these roots is always more or less obviously reduced ; in some cases 

 the root-cap is entirely devoid of falling starch-grains, while in others 

 the statocysts are relatively few in number, and the individual starch- 

 grains unusually small. The fact that the ageotropic aerial roots 

 of certain root-climbers (Hcdcra helix, Marcgravia clubia, Hoya 

 carnosd) contain no starch -grains, or, at most, such as are immovable, 

 also accords with the statolith theory. The grasping-roots of Aroids 

 are likewise only slightly geotropic or indifferent ; most of them 

 accordingly possess a more or less extensively reduced statolith- 

 apparatus, as Tischler and Gaulhofer have shown 304 ; their statocysts are 

 few in number, and their starch-grains small and sluggish or motionless. 

 The close correlation which prevails, among roots, between the degree 

 of geotropic sensitiveness on the one hand, and the development of 

 the statolith-apparatus on the other, has been very fully investigated 

 by Tischler. 305 This author states, that the root-caps of permanently 

 ageotropic terrestrial roots (e.g. in Arum maculatum, Salix spp., 

 Epimedium alpinum, Carex arenaria, etc.) contain either no starch- 



