The following link is to a paper presented at the Annual Meeting of the Society of Experimental Biology on recent experimental evidence which shows that signals are sent by the root system of potted plants to the leaves to limit the rate of growth as soon as the pot-side barrier is encountered. The paper also observes that the majority of roots on potted plants are located near the outside of the pot. This aspect is well understood by experienced cymbidium growers who realise that in order to develop a tight root ball the plants need to be started off in small pots and potted-on at regular intervals. The experiment reported on used MRI techniques to track root development.
http://www.bbc.co.uk/nature/18612661
