This phenomenon of stripping is called girdling and is used to kill trees. Unintentionally it also occurs when animals such as deer desperate for food in the winter eat bark and then kill off the trees. In 1800s Theodore Hartig, a German botanist studied the same phenomenon. Through an anatomical analysis he studied the cells in the inner bark which appeared to carry the sap and called them sieve tube members. Physiologists in this century used aphids to gain more insights on phloem flow.
Aphids insert their proboscis into phloem when feeding.
A number of ants utilize these 'pipe line' effect feeding off of the exudate of the ants. More recent studies included the use of radioisotope tracers. Labeled C 14 O 2 gas is given to the plants. Once incorporated into sugars, the flow of these same sugars can be followed through the plant from mesophyll to sink sites, and through the sieve tubes themselves. Many different models have been proposed for the movement of sugars through the plant 1.