Fig. 5.3-4. Transverse section through the seed coat of a mature bean seed. The band of dark red is composed of thousands of narrow columnar sclereids. Because they are columnar, they are macrosclereids. These cells have each deposited such thick secondary walls that there is virtually no lumen left in any, and all the cells died as the seed matured. Each cell is much taller and narrower than the parenchyma cells immediately below the sclereid band. In contrast to the density and toughness of the sclereids layer, the inner part of the seed coat consists of aerenchyma.