The hypothesis, that some plants will interact well with one another and increase growth rate whereas other plants may not get along well together and delay growth, is proven to be true.
Companion planting on farms or in your garden is an important part of pest control management. It is a way of allowing nature to play its role in bringing balance to the ecosystem. Companion planting reduces the need to use chemical insecticides. It also has other benefits like assisting in pollination and nitrogen fixation in the soil.
What would happen if the experiment was continued over a longer period?
The experiment can be repeated using different plants like radish, potato and peas.
Companion planting - http://en.wikipedia.org/wiki/Companion_planting
Companion planting - http://www.no-dig-vegetablegarden.com/companion-planting.html