why don't you try celery (Apium graveolens),
it's quite durable since it grows erectly.
this is biennial vege, so it won't consume much time to get it big enough

Frits Warmolt Went used oats.

From the link below: "Went used this decapitated oat coleoptile technique to demonstrate that phototropism was linked to and probably caused by a photo-induced movement of auxin from the illuminated to the shaded side of the coleoptile tip without significant destruction of the molecule. His thesis (1928) became an instant classic..[http://www.amazon.com/experimental-control-plant-growth/dp/B0007E3L1Y]

Here is a link that might be useful: Frits Warmolt Went

I forgot to mention that using Went's method of the coleoptile tip would work with your 35mm film containers while fast growing vines like beans and peas which show extreme fast phototropic and geotropic responses will hard to grow in such small containers.

Most university libraries should have a copy of Went stashed somewhere. Are you near a university?

Try weeping willow.

Arabidopsis thaliana is the 'standard' plant for growing tests.