GROWTH AND MOVEMENT 125 



region of most rapid growth, will resume its position pointing upwards, 

 while the root turns downwards (Fig. 87). The growing parts thus 

 readjust themselves in relation to gravity, and if then left undisturbed 

 till growth stops, the curvatures be- 

 come permanent. Such movements 

 are styled movements of geotropism, 

 Parts which curve towards the earth's 

 centre are called positively geotropic ; 

 those in the opposite direction nega- 

 tively geotropic. 



That gravity is the stimulating 



r ,. .... FIG. 87. 



cause of the readjustment is indicated Seedling of Bean; after having been . 

 by experiments with the Klinostat, 

 which is a clockwork arrangement for 



Slowly rotating the plant during r ? ot paving curved downwards, and the 



& plumule upwards. The former is positively, 



growth. If the plant be placed With the latter negatively geotropic. (Dr. J. M. 



. . Thompson.) 



its axis horizontal, and one slow 



rotation be completed in every quarter of an hour during the experi- 

 ment, there will be no curvature, though gravity will have been 

 acting on the horizontal shoot all the time. The explanation is that 

 the stimulus of gravity, being distributed equally on all sides, they 

 all react equally, and the part grows straight on. When seeds are 

 germinated on the revolving klinostat the roots and shoots take 

 haphazard directions, the influence of gravity upon them having been 

 neutralised by its equal distribution on all sides. 



In the case of the root it can be shown that the stimulus of gravity is received 

 at the extreme tip, and that it is handed on to the point of most rapid growth, 

 where the reaction takes place. It has been suggested that the stimulus 

 is received by means of starch-grains, which are present in the tissues 

 of the root-cap. (Fig. 87 bis, A, B, C.) These ponderable bodies may be seen 

 to fall to whatever side of the cell is lowest. When the root points down- 

 wards, and they rest on the acroscopic side of each cell, no stimulus is received. 

 The root is quiescent, and its growth is not disturbed.. But if the axis of the 

 root be so placed as to be oblique or horizontal, the starch-grains are seen 

 to fall in a short time to that side of the cell that is lowest. Their presence in 

 this unaccustomed position is believed to stimulate the protoplast. The 

 stimulus is conveyed, probably by the continuous protoplasmic system, to the 

 region of greatest growth, some distance back from the tip. There the equality 

 of growth is upset, so that by greater growth on the upward side and less on the 

 lower, a curvature is produced which restores the root-tip to its normal vertical 

 position. Similar starch-grains to those of the root-tip are found in the tip 

 of the cotyledon of certain Grasses which are also sensitive tp gravity. The 

 starch so constantly found in the endodermis of young stems is believe^ to 

 serve a similar purpose in relation to the geotropic stimulus received in the 



