Oak root. Light micrograph of a section through a secondary root from an oak (Quercus sp.) tree. The primary cortex (outer layer) has been shed by the formation of a circular meristem, the periderm, which forms an outer layer of cork (phellum, black). Under the cork layer is a thin cortex (blue) and rings of several phloem layers (light blue) with patches of fibres (red). Next is the ring of cambium tissue (light blue), which contains an outer layer of phloem vessels and an inner layer of xylem (centre). The wood is classified as diffuse-porous, with large vessels and small cells of xylem parenchyma (light red). Six big rays (light blue) and small rays (blackish) of primary xylem tissue radiate from the centre. Magnification: x7 when printed 10.
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