GROWTH AND MOVEMENT 



125 



*ion of most rapid growth, will resume its position pointing upwards, 

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

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

 11 growth stops, the curvatures be- 

 )me permanent. Such movements 

 re styled movements of geotropism. 

 5 arts which curve towards the earth's 

 ;ntre are called positively geotropic ; 

 those in the opposite direction nega- 

 tively geotropic. 



That gravity is the stimulating 

 luse of the readjustment is indicated 

 experiments with the Klinostat, 



FIG. 87. 



Seedling of Bean ; after having been 

 germinated, and the radicle had grown 

 downwards and plumule upwards, its posi- 



hich is a clockwork arrangement for tion was changed so that they were hori ' 



zontal. 



owly rotating the 



The figure shows its recovery, the 

 root having curved downwards, and the 

 plumule upwards. The former is positively, 

 the latter negatively geotropic. (Dr. J. M. 

 Thompson.) 



. 



rotating the plant during 



owth. If the plant be placed with 

 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 



cutralised 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- 

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



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

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

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



is 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 to gravity. The 

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

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



