In transverse section, they usually occur as polygonal cells and lignified walls.
British Dictionary definitions for polygonal
a closed plane figure bounded by three or more straight sides that meet in pairs in the same number of vertices, and do not intersect other than at these vertices. The sum of the interior angles is (n–2) × 180° for n sides; the sum of the exterior angles is 360°. A regular polygon has all its sides and angles equal. Specific polygons are named according to the number of sides, such as triangle, pentagon, etc
(pŏl'ē-gŏn') A closed plane figure having three or more sides. Triangles, rectangles, and octagons are all examples of polygons. ◇ A regular polygon is a polygon all of whose sides are the same length and all of whose interior angles are the same measure.