STIMULATION AND ITS EESULTS 381 



soil. A few organs, among which may be mentioned certain 

 rhizomes and the runners of many plants, grow at right 

 angles to the direction of gravity. When one of these is 

 placed at an angle from the position which it usually 

 assumes, a curvature of the growing organ results, which 

 lasts till the normal attitude is regained. Thus when a 

 young seedling is detached from the earth and laid upon 

 its side, the stem gradually curves through an angle of 

 90 and becomes erect, while the young root curves in the 

 opposite direction till it points vertically downwards. 

 Similarly when a runner is placed vertically, its apex is 

 slowly deflected till it again grows parallel with the soil. 

 These movements are termed apogeotropic, geotropic, and 

 diageotropic respectively. 



To prove these movements to be responses to the 

 stimulus of gravitation it is necessary to eliminate the 

 action of the latter force, and to observe the direction of 

 growth under the new conditions. This can be done by 

 causing the plant to grow supported upon an apparatus 

 known as a Klinostat, one form of which, devised by 

 Mr. Horace Darwin, is shown in fig. 158. The plant, 

 growing in a flower-pot, is fixed in a wooden box B, which 

 is secured by a thumb-screw tli to the plate pi ; the box 

 is cubical in form and can be fixed either as shown in the 

 figure, or with the axis of the pot at right angles to the 

 spindle ~k of the klinostat. The plate pi is attached to 

 this spindle, which ends in a point turning in the upper 

 end of the left-hand support s. The spindle is also sup- 

 ported at g on the friction wheel fr. The spindle (with 

 the plant attached) is made to rotate by means of a band 

 of silk dr, passing round the wheel w, and also round a 

 pulley on one of the axles of an American watch-action 

 clock c, which is attached by means of the screw E to 

 the support s. By passing the driving-gear over the large 

 pulley TF, the spindle is made to rotate once in thirty 

 minutes. By arranging wheels of different sizes at this 

 point, the period of rotation can be made longer or shorter. 



