56 BOTANY 
An ingenious contrivance, the clinostat (Figs. 33-34) 
has been devised to prove that geotropy is due to the 
foree of gravity. 
Obtain a cheap clock with strong works. Strength 
rather than accuracy is needed. Get a watchmaker 
to replace the axle to which the minute hand is 
attached, with a spindle carrying at its extremity a 
light metal plate. Obtain a light glass jar as shown 
in the figure, and a cork to fit it. To one side of the 
cork fix several layers of damp blotting paper, and 
to this pin young pea seedlings with their radicles 
pointing towards the circumference, and _ their 
plumules towards the centre. Cork the jar, with the 
side bearing the peas inwards, and then fix the jar to 
the spindle of the clock by means of drawing pins 
pushed into the cork and overlapping the metal 
plate with their edges. Set the clock going, and have 
as a control another jar carrying seeds in a similar 
way but not rotating. In the first jar both roots and 
_ stems will continue in their original directions, while 
in the second the roots will turn downwards and the 
stems upwards (Fig. 34). Of course, the result shown 
in the drawings is an ideal rarely, if ever, attained. 
It would appear, nevertheless, that it is the force of 
gravity that causes the root to travel towards, and 
the stem to travel away from the centre of the earth. 
When any particular seedling was being carried up 
the left side of the clock the force of gravity exerted 
a pull on its root and evidently a repulsion on its stem. 
When, however, the same seedling was travelling down 
the right side, though gravity exerted the pull and 
repulsion in the same directions as before, the seedling, 
it must be remembered, had been turned over, so that 
the effect of the force would be to correct the tendency 
to bend acquired in the first half revolution. Thus 
when the influence of gravity is eliminated or neutral- 
ized there is neither positive nor negative geotropy, 
