Auxins are hormones which are naturally found on plants. Auxins are mostly made in the tip of the roots and shoots but can diffuse to other parts of the roots or shoots. If a high concentration of auxins is produced or given to the plants then the cells in the shoots grow more but the cells in the roots grow less.
In a shoot the shaded part contains more auxins than the part with light shining onto it because the light destroys the auxins. This causes the shaded part to grow longer which in turn causes the shoots to bend towards the light. In the roots the shaded part still has more auxins but instead the shaded part grows less than the lit part. This causes the plant to bend away from the light. If the plant has not shoots then no auxins can be made meaning they will not grow. Phototrophism increases survival rate for plants because the more auxins the shoots have then the more the plant will grow. The plant turn towards the light where it can photosynthesis more.
If the tips are covered then this means the plants will grow straight because there is an equal concentration of auxins on both sides- the tip is will not be effected by the sunlight. If the shoots are left natural they will bend towards the light. An experiment to show this is to place 3 plants with shoots in a box with a source of a light only at one side and observing how different shoots grow. The 3 plants will have different shoots- one with no tips, one with covered tips and one with natural tips.