Biology XI:6 Anatomy of Flowering Plants(6 Products of Secondary Gr

Products of Secondary Growth Spring Wood and Autumn Wood The activity of cambium is under the control of many physiological and environmental factors. In spring season, cambium is very active and produces a large number of xylary elements with wider vessels. This wood is called spring wood or early wood. It is lighter and has lower density. In winters, xylem is less active and forms fewer xylary elements with narrow vessels. This wood is called autumn wood or late wood. It is darker and has higher density. Annual ring: When two kinds of wood appear in alternate concentric rings; these help us to estimate the age of a tree Heart wood and Sap wood Heartwood: Composed of dead elements, with highly lignified walls Imparts dark brown colour to the older trees as organic compounds such as tannins, resins, essential oils, gums, etc., are deposited on it Makes the plant durable and resistant to attack by microbes and pests Non-conducting Sapwood: Constitutes the peripheral region of the secondary xylem Lighter in colour Non-conducting Cork Cambium (Phellogen) As secondary growth starts, the outer cortical and the epidermis layer break down. These are replaced by another meristematic tissue called the cork cambium. Made of narrow, thin-walled, rectangular cells, and is a couple of layers thick Leads to the formation of cells on both sides: Outer cells − Differentiate into the cork (Phellem) Inner cells − Differentiate into the secondary cortex (Phelloderm) Phellogen + Phellem + Phelloderm = Periderm Phellogen exerts pressure on its peripheral layers, and due to this, these layers die. Cork has suberin deposits on the cell wall. This makes it impervious to water. Bark:Tissues exterior to the vascular cambium At some regions, the phellogen cuts off closely arranged parenchymatous cells, which rupture to form lens-shaped openings called lenticels. These openings permit the exchange of gases.