The test to determine if light is the impetus which moves plants in a particular direction is a fun one, though since most of us know of photosynthesis or remember doing the experiment in second grade, it makes it a bit less of a challenge when proving it’s effect. For one, the obvious answer would be to pick the method which puts the light and direction in multiple variables. However, just as important would be to prove that all the other components, ceteris paribus, have no real effect on the plants. For one, I would seek to disprove that which is not true, the most obvious being the possible attraction of plants to glass. By placing the plants in unlit and lit settings with glass on one side only, one would see from the non-directional or random array of the plants faces that the presence of glass would have little or no effect on the plants, let alone actually have the drive to force plants to grow towards them. After this, one would only need to prove that light in and of itself is a primary puller of the plants, so one would merely have to set up multiple experiments. The First would be with the light situated on the right. When the plants grow in that uniform direction, one would simply move the light around, with different plants to see if they grow to that side. After trying multiple directions, one might even be tempted to see if intensities of light make a difference, perhaps setting a gauzy curtain on one window, and an unveiled window the next. One would assume that the unveiled window might have greater pull on the plants, but that would be a recommendation for further experiments.
The steps described above follow the scientific in that all of them are quite observable and documentable. Control groups are also easily implemented with the state of merely being left by themselves in a dark room. Also, it is highly easy to replicate, reproduction of the experiment being highly necessary, especially with the oft used bean plants, which grow...