In a transverse section of a sunflower root :
Answers
The image shows a transverse section through part of a young stem of Sunflower (Helianthus annuus) containing one vascular bundle. Sunflower is a dicot. At this stage the stem is about 10 mm in diameter, and the large vascular bundles are still separate, with secondary thickening having not yet taken place.
Beneath the epidermis is a layer of collenchyma tissue about 7 to 8 cell layers thick. Collenchyma consists of living cells with unevenly thickened cellulose walls. It provides extra support in young, growing stems, undergoing stretching as the stem grows.
Outside the vascular bundle is a large cap of fibres (sclerenchyma). This tissue is also for support, but the cell walls are made of cellulose impregnated with lignin, producing a tissue that is more rigid.
The large red xylem vessels are easily distinguished in the lower half of the vascular bundle, surrounded by xylem parenchyma cells.
Above the xylem is a region of 5 to 6 layers of undifferentiated cells that contain the cambium, and above this, below the sclerenchyma, is the phloem.
A mature sunflower plant can grow to 2 to 3 metres in height, and the stem may have to support a flower head 30 cm in diameter. The primary stem structure shown here would not be able to support such a plant. To reach this size secondary thickening has to take place, with the laying down of woody tissues in the growing stem.
The width of the vascular bundle is about 700 µm.
