All the stems of plants and trees are green in colour when young. But why does the stem colour change to brown as they grow older?
The stem of all plants is green due to the presence of hypodermal chlorenchyma in its cortex. But when the stem axis, especially the dicot stem axis, ceases to elongate and undergoes secondary growth. Its green colour disappears and becomes pale or dark brown in colour.
The secondary growth is not only responsible for the change in surface colour but also contributes to the thickness of the axis due to the formation of the periderm. This is effected in the cortex by a lateral meristem called phellogen and the formation of secondary vascular tissues, which is effected in the stale (vascular cylinder) by the vascular cambium.
Among these two tissue gains due to secondary growth, the periderm is responsible for the change in stem colour. The periderm is the protective tissue of secondary origin replacing the primary structure of the young stem, the epidermis.
It consists of the phellogen, the meristem that produces the periderm, the phellem, the protective tissue formed outwardly by the phellogen. The phelloderm a living parenchyma tissue, is formed inwardly by the phellogen.
In a stem, the periderm is most commonly formed in the sab-epidermal layer. In some species, however, the first periderm appears rather deep within the stem. All the living tissues above the periderm die due to the insertion of the nonliving cork between these tissues and the living inner tissues of the plant.
Now the stem surface is exposed by the cork cells. The walls of the cork cells may be coloured brown, yellow or the lumen of these cells may contain resinous or titaniferous materials. That is why the older stems exhibit brown colouration on their surface.