A student examines a cross section of a mature Equisetum stem. Which structures would she see, starting at the center of the stem and moving outward? a: Central canal carinal canals vallecular canals epidermis b: Pith vallecular canals central canals epidermis c: Pith vallecular canals carinal canals epidermis d: Central canal vallecular canals carinal canals epidermis.