Question

What is the difference between primary and secondary phloem?

Answer 1

Transportation of necessary nutrients and water in plants is carried out by two tissues: phloem and xylem. The vascular tissue phloem carries carbon (photosynthates) from the leaves to the plant's basal portions, while the undifferentiated tissue vascular cambium is responsible for secondary growth and healing of damaged phloem. Phloem contains specialized sieve tube cells which help in translocation of nutrients across the plant body.

Complete answer:
The difference between primary and secondary phloem is tabulated below:

Primary phloem	Secondary phloem
It is made up of the apical meristem's procambium.	The vascular cambium is a lateral meristem that produces secondary phloem.
All vascular plants have it in their main plant body.	With the exception of annuals, it is exclusively seen during secondary growth of dicots and gymnosperms.
Primary phloem can be found in every organ.	Secondary phloem is only found in perennial dicots and gymnosperms' stems and roots.
It is found on the periphery of the plant bodies.	It develops within the main phloem.
There is no radial system.	The radial system of phloem rays traverses it.
There are fewer phloem fibres. They are only found in the outlying areas of the plant body.	The phloem fibres are more plentiful in numbers. Patches or bands of them are frequent.
The primary phloem has an uneven arrangement of several cell types.	The arrangement of secondary phloem is more uniform.
There are fewer sieve tubes found in it.	There are more sieve tubes found in it.
Sieve tubes are longer and thinner.	Sieve tubes are thicker and shorter.
The phloem parenchyma is scarce.	Phloem parenchyma is abundant in it.

Note:
Secondary growth occurs as a consequence of cell division in the cambia or lateral meristems, causing stems and roots to thicken, whereas primary growth happens as a result of cell division at the ends of stems and roots, forcing them to lengthen and giving rise to primary tissue. Secondary growth in plants helps in the formation of thick woody stems in angiosperms and many gymnosperms.