planting spuds

To get slightly technical:

Plants don't grow towards light per se, they stop growing straight. Sunlight reduces the production of auxins which are the hormones which make the plant tip grow. So one side of the plant has reduced growth the other side more growth so the plant bends towards the light. This is phototropism.

Underground there is obviosly no light so there is no reduction in auxin production so the root tip keeps growing at its fullest rate in a straight line. Here geotropism takes over and causes the plant tip to grow vertically. Once the tip breaks the surface then phototropism is more dominant.

No one has mentioned hydrotropism which causes the roots to grow towards moisture.I knew O-level biology would be useful one day.