Phloem is responsible for transportation of food from source to sink.
The organ where food is synthesised or stored is called source. Leaves act as a sink for a considerable period. In some plants, the roots act as sink during autumn period.
The organ where food is utilized, such as roots, flowers, fruits, branches, etc. are called sink.
On the basis of position, the phloem is of two types:
External phloem: It is the usual phloem present outside the xylem.
Internal or intraxylary phloem: It is present internal to the xylem. It is seen in the members of Apocynaceae, Asclepiadaceae, Convolvulaceae, Solanaceae.
Induced or Interxylary phloem: It is the secondary phloem that occurs in patches within the secondary xylem. For example, Leptadaenia, Salvadora, Chenopodium, Boerhaavia, Amaranthus.
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External phloem is the usual phloem present outside the xylem.