Name the motor to which the cork was attached. Name the response shown by the seedlings in apparatus P. Explain the mechanism that caused the roots in apparatus P to grow downwards.

The motor in apparatus Q continuously turns the seedlings to distribute the gravitational stimulus evenly. This device is used to counteract the effect of gravity on plant growth.

a) (i) The motor to which the cork was attached is a:
$\boxed{\text{Clinostat}}$

In apparatus P, the roots grew downwards, which is a growth response towards gravity.

a) (ii) The response shown by the seedlings in apparatus P is:
$\boxed{\text{Positive gravitropism (or positive geotropism)}}$

Roots exhibit positive gravitropism due to the uneven distribution of the plant hormone auxin.

a) (iii) The mechanism that caused the roots in apparatus P to grow downwards is:
Gravity causes auxin to accumulate on the lower side of the root. In roots, a high concentration of auxin inhibits cell elongation. This means the cells on the upper side of the root elongate more than those on the lower side, causing the root to bend and grow downwards.