In sunflower stem, after a period of growth of one year secondary development takes place. Secondary growth is seen both in the vascular cylinder as well in the cortex.
Secondary growth in the vasculature is brought about by the activity of a fasicular cambium. In the primary structure the cambium is in the form of strips present in between xylem and phloem.
At the initiation of secondary growth, the parenchymatous cells of the medullary rays become meristematic. As a result, new strips of cambia are formed in between the vascular bundles adjacent to the intrafascicular cambia. The newly formed strips join with the intrafascicular cambia and form a completely closed ring.
At the beginning of secondary growth each cambial cell divides trangentially and forms two cells. Of these, one remains meristematic while the other differentiates either into the secondary xylem element or a secondary phloem element depending on its position.
If the differentiating cell is towards the periphery it forms the secondary phloem or if it is towards the pith, it forms the secondary xylem. Because of the addition of secondary vascular tissues the girth of the stem increases. Generally more secondary xylem is formed than secondary phloem. As a result the cambium is pushed towards the periphery.
Annual rings or growth rings:
The activity of the fascicular cambium is more periodical than continuous throughout the year. As a result of this periodic activity, distinct growth layers are formed in the secondary xylem. In a transverse section of the stem, these layers appear as rings, hence called annual rings or growth rings. An annual ring the xylem consists o two types of wood elements namely spring wood or early wood and autumn wood or late wood.
During spring season growth is maximum and hence additional vessels are needed to supply the nutrition. Hence spring wood is more in extent and has vessels with a wider lumne.
During autumn season, there is less growth and hence less amount of vessels are formed with narrow lumne. The spring and autumn woods of successive rings alternate. Since each annual ring corresponds to the growth during a year, the number of rings corresponds to the age of a particular plant after its initial growth of one year.
Sap wood and Heart wood: After a considerable period of secondary growth, the secondary xylem gets differentiated into two regions. The central region consisting of old wood is called the heart wood or duramen. The peripheral zone of wood or the recently formed wood is called the sap wood or albumum.
While the heart wood has mostly dead cells, sap wood has some living cells in association with that of stele. In the cortex, a cork cambium or phellogen arises in the outer layers and cuts off derivatives towards both the faces. The outer derivatives form the cork or the phellem while the inner derivatives form the phelloderm or the secondary cortex. These three together constitute the periderm tissue which replaces the epidermis.
In older stems, the living tissues are completely covered by the dead cork and as a result the communication with the exterior is cut off. In order to bring communication with the exterior, here and there on woody stems, small openings are formed called lenticles. Lenticles function like stomata. They are formed immediately below the stomata and may be scattered or arranged in vertical or horizontal rows.