UPWARD GROWTH OF THE HEAD, AND [Part II. 
But an admirable experiment of Knight’s 
furnishes the main fact from which it has been 
asserted that both the ascent of the stem and 
the descent of the root should be referred to 
gravity. When he subjected beans to a strong 
centrifugal force by making their seeds grow 
on the rims of wheels whirled rapidly by water, 
their roots grew from the centres of the wheels, 
and their stems towards the centres of the wheels. 
When the wheel was vertical, the growth of the 
plants was precisely as stated. When the wheel 
was horizontal, the growth of the plants was 
nearly horizontal; but the stems inclined up¬ 
ward and the roots downward in an inverse 
ratio as the degree of centrifugal force. Richard 
tells us that this experiment was repeated by 
Dutrochet, and the only difference was, that, in 
the case of the horizontal wheel, “ the inclination 
was much greater, and the radicles and gem- 
mules had become almost horizontal.” This last 
witness appears to me to prove too much: for, 
granting the effects of gravity on the growth of 
plants to cease directly as the centrifugal force 
applied, the centrifugal force applied to each 
part of the plants in this experiment would 
it is green, the colour must be owing to the action of light, 
independently of atmospheric air. 
