The endodermis in the stem of dicot plants is made up of parenchymatous cells and it stores the starch. Therefore, it is also called the starch sheath. When stained with iodine, the endodermis will take up the blue colour due to the presence of starch. Hence, the correct answer is 'Endodermis'.

Staining of Dicot Stem with Iodine

Iodine staining is a common technique used in plant biology to visualize the presence of starch in plant tissues. When iodine solution is applied to a dicot stem, certain structures are expected to show a blue color. Let's discuss each of these structures and why the correct option is (c) endodermis.

Pericycle
- The pericycle is a tissue layer located just inside the endodermis in dicot stems.
- It is responsible for the development of lateral roots.
- The pericycle does not contain significant amounts of starch, so it will not show a blue color when stained with iodine. Therefore, option (a) is incorrect.

Cortex
- The cortex is the region of the stem located between the epidermis and the vascular bundles.
- It acts as a storage tissue and may contain starch granules.
- When stained with iodine, starch-containing cells in the cortex may appear blue or purple.
- However, the cortex does not always contain starch, so the presence of a blue color in the cortex is not guaranteed. Therefore, option (b) is not the correct answer.

Endodermis
- The endodermis is a single layer of cells surrounding the vascular tissue in dicot stems.
- It plays a crucial role in regulating the movement of water and minerals into the vascular tissue.
- The endodermis is known to contain starch granules in its cells, particularly in the Casparian strips.
- When stained with iodine, the starch-containing cells of the endodermis will turn blue or purple.
- Therefore, option (c) is the correct answer.

Phloem
- The phloem is the tissue responsible for the transport of organic nutrients, such as sugars, throughout the plant.
- It does not usually contain significant amounts of starch.
- Therefore, the phloem is not expected to show a blue color when stained with iodine. Hence, option (d) is incorrect.

In summary, when staining a dicot stem with iodine, the endodermis is expected to show a blue color due to the presence of starch in its cells. The pericycle, cortex, and phloem are not expected to show a significant blue color.