The outermost layer is the protoderm, which differentiates into the epidermis, then comes the procambium, which gives rise to the primary vascular tissues (primary xylem and primary phloem), and finally the ground meristem, precursor of ground tissues (parenchyma, collenchyma and sclerenchyma).

Primary phloem develops from the provascular tissue. The provascular tissue is a type of meristematic tissue that differentiates into primary vascular tissues - primary xylem and primary phloem. The development of primary phloem occurs as part of primary growth in plants.

Explanation:
1. Meristems and primary growth:
- Meristems are regions of undifferentiated cells that are responsible for plant growth.
- They give rise to various types of tissues and organs in plants.
- Primary growth refers to the increase in length of the plant body.
- It occurs at the apical meristems located at the tips of the shoots and roots.

2. Provascular tissue:
- Provascular tissue is a type of meristematic tissue located in the apical meristem.
- It gives rise to the primary vascular tissues - primary xylem and primary phloem.
- Provascular tissue is also known as procambium.

3. Development of primary phloem:
- During primary growth, the provascular tissue differentiates into primary xylem and primary phloem.
- The primary xylem develops towards the inner side, while the primary phloem develops towards the outer side of the stem or root.
- The primary phloem is responsible for the transport of organic nutrients, such as sugars, from the leaves to other parts of the plant.

4. Differentiation of phloem cells:
- The cells of the primary phloem differentiate into different cell types.
- Sieve tube elements are the main conducting cells of the phloem.
- They are elongated cells with perforated end walls called sieve plates, which allow the flow of sap.
- Companion cells are associated with sieve tube elements and provide metabolic support.

In conclusion, primary phloem develops from the provascular tissue, which is a type of meristematic tissue located in the apical meristem. The provascular tissue differentiates into primary xylem and primary phloem, with the primary phloem developing towards the outer side of the stem or root. The primary phloem is responsible for the transport of organic nutrients in plants.

Provascular tissue.