Question

During the secondary growth in a dicotyedonous stem, the fusiform initials of vascular cambium give rise to (a) secondary xylem (b) secondary phloem (c) medullary ray (d) both (a) and (b).

Short answer

The fusiform initials of vascular cambium give rise to both secondary xylem and secondary phloem, making choice (d) both (a) and (b) correct.

Step-by-step solution

Understanding Secondary Growth

Secondary growth in dicotyledonous stems refers to the increase in the girth of the plant stem, and it happens due to the activity of the vascular cambium. Vascular cambium is a lateral meristem, which means it contributes to the growth in thickness rather than length.

Recognizing the Role of Fusiform Initials

Fusiform initials are elongated cells in the vascular cambium that divide and contribute to the formation of new cells. When the fusiform initials divide, they give rise to cells that differentiate into secondary xylem (wood) towards the inside of the stem and secondary phloem (part of the bark) towards the outside.

Identifying the Correct Options

Since fusiform initials give rise to secondary xylem and secondary phloem through their differentiation and division, the correct answer includes both of these products of secondary growth.

Key concepts

Vascular Cambium

The vascular cambium is one of the primary drivers behind the thickening of dicotyledonous stems, a process known as secondary growth. Unlike the apical meristem which facilitates growth in length, the vascular cambium is a lateral meristem which means it's responsible for increasing the girth of the stem. Located between the primary xylem and the primary phloem, this narrow layer of cells has the unique ability to divide and differentiate, leading to the formation of new tissues.

During the growth cycle, the vascular cambium gives rise to secondary xylem on the inner side and secondary phloem on the outer side, contributing to both the hardness of the wood and the versatility of the bark. This increased girth strengthens the plant and allows it to better support itself as it matures. The function and continual growth of vascular cambium are essential not only for the plant's structure but also for its ability to transport water and nutrients.

Secondary Xylem

Secondary xylem is the result of the vascular cambium's inward differentiation and is a major component of wood in dicotyledonous plants. Its formation marks a significant stage in the plant's development, as it strengthens the structural integrity and provides a pathway for water and mineral transport from the roots to the leaves.

As the plant ages, secondary xylem accumulates and is at times referred to as heartwood when it becomes non-functional, adding girth and support to the stem. Living xylem, conversely, also called sapwood, continues to play an active role in the transport system of the plant. Understanding the crucial role of secondary xylem in a plant's vascular network sheds light on how trees and similar plants are able to grow both in height and diameter over time.

Secondary Phloem

Secondary phloem, much like secondary xylem, is a byproduct of the vascular cambium's activities but is formed on the stem's outer side. This tissue is a vital part of the plant's circulation system, responsible for transporting nutrients and sugars that are created by photosynthesis in the leaves to other parts of the plant where they are needed.

Over time, as secondary growth continues, older secondary phloem layers get crushed and incorporated into the bark, which provides protective functions for the plant. The continuous formation of secondary phloem is crucial for the plant's sustenance and growth since it supports the plant's metabolic activities by distributing the necessary organic compounds throughout the plant.

Fusiform Initials

Fusiform initials are specialized cells within the vascular cambium. They are notably elongated and spindle-shaped, which is where they get their name: 'fusiform' means spindle-like. These cells are the true workhorses of secondary growth, as they divide longitudinally and differentiate to become either secondary xylem or secondary phloem.

The process is complex as the fusiform initials have to maintain their population while providing the ever-growing plant with new tissues. Dividing both periclinally (parallel to the surface) and anticlinally (perpendicular to the surface) allows them to sustain the cambium and contribute to the increasing circumference of the plant. Recognizing the pivotal role of fusiform initials helps in understanding the dynamic process of plant growth and the intricate balance of cell division and differentiation required for a plant's development.