Sources.

The areas of sources include any exporting organ typically a mature leaf, or storage organ, that is capable of storing photosynthate in excess of its own needs.

Sinks.

Sinks are the areas of active metabolism or storage for example roots, tubers, developing fruits, immature leaves, and even the growing tips of stem and root.

Storage organ

It is during the exporting phase of its development. In biennials e.g. root of beet is a sink in first growing season, but becomes source in the next growing season, when sugars are utilized in growth of new shoots. The movement in phloem is from source to sinks in most of the plants during active photosynthesis. Composition of Phloem Sap (Honey Dews) Method to study. The composition of materials flowing in phloem has been studied by using aphids – the insects which are phloem feeders. These insects insert their stylets into stem or leaf and extend them to puncture a sieve tube. The pressure in the sieve tube cell forces sap through aphid‟s digestive tract and out its posterior end as droplets called “honey dew”. Composition. The compositions of honey dew have revealed that it contains 10-25% dry matter 90% or more of which is sucrose. Nitrogenous compounds are about 1%.