Describe the distribution of xylem and phloem in transverse sections of stems, roots and leaves of herbaceous dicotyledonous plants

Sourav PanOctober 29, 2024

Answered

The distribution of xylem and phloem in the transverse sections of stems, roots, and leaves of herbaceous dicotyledonous plants is characteristic and can be summarized as follows:

1. Transverse Section of Stems

• Arrangement: In herbaceous dicot stems, xylem and phloem are organized in vascular bundles arranged in a ring or a circle.
• Xylem: The xylem is typically located towards the center (inside) of the vascular bundle and is often larger in diameter than the phloem. It appears as dark-stained regions in sections due to lignification.
• Phloem: The phloem is situated towards the outside of the xylem in the vascular bundle. It appears lighter in color and consists of sieve tube elements and companion cells.
• Cortex: Outside the vascular bundles, there is a layer of cortex made up of parenchyma cells, which provides storage and support.

2. Transverse Section of Roots

• Arrangement: In herbaceous dicot roots, xylem and phloem are also found in vascular bundles, but their arrangement differs from that in stems.
• Xylem: The xylem is located at the center of the root in a star or cross shape. This arrangement creates a central region of xylem tissue that appears as a series of arms radiating from the center.
• Phloem: The phloem is found in smaller bundles located between the arms of the xylem. It appears as lighter-stained tissue in the cross-section.
• Endodermis: Surrounding the vascular tissue is the endodermis, a single layer of cells that regulates the movement of water and nutrients into the vascular system. The pericycle lies just outside the endodermis and can give rise to lateral roots.

3. Transverse Section of Leaves

• Arrangement: In leaves, xylem and phloem are located within the vascular bundles, which can be arranged in various patterns depending on the type of leaf.
• Xylem: In dicot leaves, the xylem is typically located on the upper side (adaxial surface) of the vascular bundle. It is positioned closer to the leaf surface.
• Phloem: The phloem is situated on the lower side (abaxial surface) of the vascular bundle, underneath the xylem.
• Mesophyll: The vascular bundles are embedded in the mesophyll tissue, which consists of parenchyma cells that carry out photosynthesis. The arrangement helps facilitate the transport of water and nutrients from the xylem to the mesophyll and sugars from the mesophyll to the phloem.

Summary

• Stems: Vascular bundles arranged in a ring with xylem on the inside and phloem on the outside.
• Roots: Xylem forms a central star-shaped structure with phloem located in between the arms of the xylem.
• Leaves: Vascular bundles have xylem on the upper side and phloem on the lower side, integrated within the mesophyll tissue.