Final answer:
Cellulose is the primary structural constituent of the plant cell secondary wall, laying the framework for strength and rigidity that supports plant growth and form.
Step-by-step explanation:
The primary structural constituent of the plant cell secondary wall is E) cellulose. Cellulose is a polysaccharide composed of glucose units, and it forms the backbone for the structure of the secondary cell walls in plants. In the secondary cell wall, cellulose microfibrils are embedded in a matrix of lignin, which is a complex polymer that gives the wall its rigidity and strength, particularly in woody plants. The secondary cell wall is crucial as it provides the necessary support for the plant, allowing for erect growth and resistance to the compressive forces of gravity. Lignin, which is also found in the secondary cell wall, contributes to the longevity of the plant and its evolutionary success. However, the specific component being asked for is cellulose which is laid down by enzymes and provides the structural framework of the cell wall.