Question

Which of the following statements is correct aboutar, dicot stem which shows extensive secondary growe: (a) Primary xylem persists in the centre of the aris (b) Primary and the older secondary phloem get etu (c) Secondary xylem forms the bulk of the stem (d) All of these

Short answer

The correct statement about a dicot stem with extensive secondary growth is (c) Secondary xylem forms the bulk of the stem.

Step-by-step solution

Understanding the Anatomy of Dicot Stems

In a dicot stem experiencing secondary growth, the vascular cambium forms between the primary xylem and primary phloem. Over time, it produces secondary xylem towards the inside and secondary phloem towards the outside. With extensive secondary growth, the secondary xylem can become the most prominent feature of the stem's cross section.

Assessing the Options

We evaluate each option given in relation to the anatomy of a dicot stem with significant secondary growth. (a) The primary xylem does persist in the center, often as a pith. (b) Primary and older secondary phloem do not get crushed as new phloem layers are produced towards the outside and the older layers are pushed outwards; this might imply the phloem remains intact but becomes less functional. (c) Secondary xylem indeed forms the bulk of the stem due to continuous production by the vascular cambium.

Selecting the Correct Statement

Considering the anatomy and examining each statement in light of how secondary growth occurs in dicot stems, we can evaluate the accuracy of each statement. (a) While primary xylem does remain in the center, this statement alone doesn't fully encompass the effects of extensive secondary growth. (b) This statement is not accurate, as older secondary phloem does not get crushed. (c) Secondary xylem does form the bulk of the stem during extensive secondary growth, which makes it a correct statement.

Conclusion

After analysis, the correct statement about a dicot stem with extensive secondary growth is that secondary xylem forms the bulk of the stem, which corresponds to option (c).

Key concepts

Vascular Cambium

The vascular cambium is a cylinder of meristematic cells located between the primary xylem and primary phloem in dicot stems. As a part of secondary growth, this tissue plays a critical role in thickening the stems of dicotyledonous plants. It is the source of both the secondary xylem and secondary phloem.

The cambium is actively dividing, and as it produces new cells, they differentiate into either xylem or phloem. On the inner side, the cambium produces secondary xylem (wood), and on the outer side, it produces secondary phloem (bark). Over the years, these added layers can lead to a substantial increase in the girth of the stem. The ability of the vascular cambium to form secondary vascular tissues is vital for the plant's growth and adaptation to the environment, allowing the plant to transport more nutrients and water as it increases in size.

Secondary Xylem

Secondary xylem is the tissue produced by the vascular cambium on its inner side and plays an essential role in water and mineral transportation within the plant. It is also responsible for storing nutrients and supporting the plant's structure. As secondary growth occurs, the secondary xylem accumulates and forms wood, constituting the majority of a mature dicot stem's mass.

In woody plants, the secondary xylem creates growth rings as the rate of xylem production changes with the seasons, leading to patterns of dense and lighter wood. These rings can be used to determine the age of the tree and provide information about the environmental conditions during each period of growth. The persistence of primary xylem at the stem's center is typically observed in dicot plants, which becomes encased by the ever-increasing layers of secondary xylem. This results in the primary xylem forming a sort of 'core' within the plant's woody stem.

Secondary Phloem

Secondary phloem is the tissue produced by the vascular cambium on its outer side and plays a crucial role in the transport of sugars and other metabolic products throughout the plant. Unlike secondary xylem, the secondary phloem does not accumulate to the same extent because the outer layers eventually get compressed and are sloughed off over time in a process known as exfoliation.

This exfoliation typically happens because as the stem diameter increases due to the addition of new xylem, the older phloem layers become stretched and can no longer function properly. However, this doesn't mean that older secondary phloem gets crushed as new layers are produced; instead, they become less functional as they are pushed outwards and replaced by newly formed phloem. Maintaining phloem efficiency is critical for the plant's survival, enabling the distribution of carbohydrates produced in the leaves to the rest of the plant, particularly to zones of active growth like roots and new shoots.