380 VEGETABLE PHYSIOLOGY 
air; primary roots grow vertically downwards into the 
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. When, for instance, 
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 (fig. 156). The plant, growing 
in a flower-pot, is fixed in a wooden box B, which is 
secured by a thumb-screw th to the plate pl; 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 & of the klinostat. The plate pl 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 ¢, which is attached by means of the screw RB to 
the support s. By passing the driving-gear over the large 
pully W, the spindle is made to rotate once in twenty 
minutes. By arranging wheels of different sizes at this 
point, the period of rotation can be made longer or shorter. 
