Do all sources supply all sinks on a plant?
What you think about phloem is not correct, actually there are a lot of phloem which transport the photosynthetic. Although the overall pattern of transport in the phloem can be stated simply as source-to-sink movement, the specific pathways involved are often more complex. Not all sources supply all sinks on a plant; rather, certain sources preferentially supply specific sinks. In the case of herbaceous plants, such as sugar beet and soybean, the following generalizations can be made.


Proximity. The proximity of the source to the sink is a significant factor. The upper mature leaves on a plant usually provide photosynthetic to the growing shoot tip and young, immature leaves; the lower leaves supply predominantly the root system. Intermediate leaves export in both directions, bypassing the intervening mature leaves.
Development. The importance of various sinks may shift during plant development. Whereas the root and shoot
apices are usually the major sinks during vegetative growth, fruits generally become the dominant sinks during
reproductive development, particularly for adjacent and other nearby leaves.
Vascular connections. Source leaves preferentially supply sinks with which they have direct vascular connections. In the shoot system, for example, a given leaf is generally connected via the vascular system to other leaves directly above or below it on the stem. Such a vertical row of leaves is called an orthostichy. The number of internodes between leaves on the same orthostichy varies with the species.
Modification of translocation pathways. Interference with a translocation pathway by wounding or pruning can alter the patterns established by proximity and vascular connections that have been outlined here. In the absence of direct connections between source and sink, vascular interconnections, called anastomoses (singular anastomosis), can provide an alternative pathway. In sugar beet, for example, removing source leaves from one side of the plant can bring about cross-transfer of photosynthetic to young leaves (sink leaves) on the pruned side. Removal of the lower source leaves on a plant can force the upper source leaves to translocate materials to the roots, and removal of the upper source leaves can force lower source leaves to translocate materials to the upper parts of the plant.