The plant used to make ancient Egypt’s primary writing material was papyrus.
Papyrus was made from the pith of Cyperus papyrus, a tall wetland sedge that once grew abundantly along the Nile. Craftspeople cut the inner pith into strips, arranged layers at right angles, pressed them together, and dried the resulting sheets. Several sheets could then be joined into a long roll.
Egyptian scribes used papyrus for administrative records, letters, literature, religious texts, and medical and mathematical works. They generally wrote with reed pens and black or red ink. The material was lighter and more portable than stone, although it was vulnerable to moisture and survived especially well in Egypt’s dry climate.
Papyrus is not the same as modern paper, though the English word “paper” ultimately traces back to the Greek term for papyrus. It was also used for boats, mats, rope, sandals, baskets, and other objects. Flax and reeds were important Egyptian plants, but neither is the canonical answer for this writing material.