GCSE/Everyday
The woodlouse will move towards moisture.
A plant would send its roots deeper into the soil to find more water.
A Level
The woodlouse would increase its movement in random directions till it found an environment with a higher level of moisture.
A plant would do more mitosis and cell division in the meristem tissue at the tips of the roots. This would result in elongation of the roots which would mean they would extend deeper into the soil and increase the chances of accessing more water.