Describe an experiment which you could use to support the theory behind positive phototropism. You will need to ‘cut’ your plant.

Phototropism is the movement of plant in response to light. Positive phototropism refers to movement towards light, while negative phototropism means movement of plant away from the light. It has been observed that leaves exhibit positive phototropism, while roots exhibit negative phototropism. The positive phototropism means that the shaded part of the leaves grow faster than the non-shaded or illuminated portion. We can observe the positive phototropism using an experiment. We can recreate the experiment conducted by Darwin and Boysen-Jensen. 

Take two pots containing grass seeds and allow them to grow. In the initial phases, only coleoptiles come out of soil and the leaves break out of these coleoptiles later on. The coleoptile is a hollow sheath covering of the leaf. In one of the pots, cover the coleoptile with an opaque covering and you will observe that no phototropism is there (no bending towards light). Remove the opaque covering and cut the tip of coleotile. In absence of the tip, there is no phototropism (in fact, the growth stopped). At the same time, you can observe the positive phototropism in the other pot (control sample). From this experiment, you can see that the tip of coleoptile has something to do with phototropism. In fact, the tip releases an auxin (a chemical that controls cell elongation in the stem) and in absence of the tip, the plant is unable to exhibit phototropism.  

Hope this helps.