Structure, Composition & Classification of Primary and Secondary Phloem
What is phloem?
Phloem is a complex tissue system in plants. It is the food conducting tissue of vascular plants. Together with xylem, they form the vascular tissue system. The phloem composed of several types of cells among which some are living cells and some are dead. The term ‘phloem’ was introduced Nageli (1853) from a Greek word ‘phloios’ meaning ‘bark’. The ‘bark’ is a non-technical term describing all tissue outside the secondary xylem of the plant. Botanically the bark includes secondary phloem, cortex, primary phloem and periderm. The current post describes the structure, composition and classification of phloem.
Location of phloem in plants:
Ø Usually, the phloem is situated external to xylem.
Ø In leaves, the phloem is located on the abaxial side (lower side).
Ø In some plants (members of Cucurbitaceae and Convolvulaceae), the phloem is present on both external and internal to xylem. Such a vascular bundle is called bicollateral vascular bundle.
Ø Phloem present internal to the xylem is called ‘internal phloem’ or intra-xylary phloem.
Ø Phloem located external to the xylem is called ‘external phloem’.