Final answer:
A desmotubule, derived from the smooth endoplasmic reticulum, is the structure typically observed within plasmodesmata linking plant cells for intracellular transport.
Step-by-step explanation:
The dense central structure that is derived from the smooth endoplasmic reticulum (SER) and usually seen in the plasmodesmata is called a desmotubule. Plasmodesmata are channels that pass between the cell walls of adjacent plant cells, linking their cytoplasm and enabling the transport of materials from cell to cell. Unlike the rough endoplasmic reticulum (RER), which has ribosomes attached to its surface and is involved in protein synthesis, the smooth ER does not have ribosomes on its surface and is primarily involved in the biosynthesis of lipids, carbohydrate metabolism, and detoxification of harmful compounds within the cell.
Both the smooth and rough ER are part of the larger endomembrane system within eukaryotic cells, which includes interconnected membranous structures that collectively modify proteins and synthesize lipids. The rough ER is covered with ribosomes and inspects proteins in cisternae for export or insertion into various cellular components. However, the smooth ER forms a network that, in the context of plasmodesmata, gives rise to the central rod-like structure known as the desmotubule, which aligns with the ER of adjacent cells.