Primary xylem in roots is exarch, meaning the protoxylem is located towards the periphery, while primary xylem in stems is endarch, with the protoxylem located towards the center. This difference in arrangement can be attributed to the different developmental processes and functional requirements of roots and stems.

Root Development and Exarch Xylem:
1. Root initiation: In the early stages of root development, the primary meristem called the promeristem gives rise to the root apical meristem (RAM).
2. Procambium differentiation: The procambium, a primary meristematic tissue, then differentiates into primary xylem and primary phloem.
3. Protoxylem formation: The first-formed primary xylem cells, known as protoxylem, are located towards the periphery of the root.
4. Metaxylem formation: As root growth continues, additional primary xylem cells are formed towards the center, known as metaxylem.
5. Exarch arrangement: The exarch arrangement of primary xylem in roots allows for efficient water and mineral uptake from the soil.

Stem Development and Endarch Xylem:
1. Stem initiation: Stem development begins with the formation of the apical meristem, which gives rise to the shoot apical meristem (SAM).
2. Procambium differentiation: Similar to root development, the procambium differentiates into primary xylem and primary phloem.
3. Protoxylem formation: In stems, the first-formed primary xylem cells, or protoxylem, are located towards the center.
4. Metaxylem formation: As stem growth continues, additional primary xylem cells are formed towards the periphery, known as metaxylem.
5. Endarch arrangement: The endarch arrangement of primary xylem in stems provides structural support and allows for efficient transport of water and nutrients from the roots to the leaves.

Functional Differences:
1. Water and nutrient uptake: Roots primarily function to absorb water and minerals from the soil, necessitating an exarch arrangement to facilitate efficient absorption.
2. Mechanical support: Stems provide structural support to the plant, thus requiring an endarch arrangement that promotes stability and strength.
3. Directionality of transport: The exarch arrangement of primary xylem in roots ensures the unidirectional flow of water and nutrients from the root towards the stem and leaves, while the endarch arrangement in stems supports bidirectional transport.

In conclusion, the exarch arrangement of primary xylem in roots and the endarch arrangement in stems are a result of their different developmental processes and functional requirements. The exarch arrangement in roots supports efficient water and nutrient uptake, while the endarch arrangement in stems provides mechanical support and facilitates bidirectional transport.

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