Secondary Growth and The Anatomy of Wood - Most monocots and many dicots show little or no secondary growth. These are often herbaceous, nonwoody plants. They usually have a life cycle that lasts one season. Corn, dandelions, black-eyed Susans are examples of herbaceous, nonwoody plants that do not have secondary growth. An herbaceous plant is made often flexible nonwoody tissue. - On the other hand, many dicots show secondary growth, which allows a plant to growth laterally or in other words to thicken their stems. Plants which show secondary growth are woody plants (possess wood in their plant bodies). Examples include trees, vines and shrubs. - Secondary growth in stems is the product of secondary meristems (Vascular Cambium and Cork Cambium). These secondary meristems give rise to our secondary tissues. - Cork cambium replaces the epidermis with a secondary dermal tissue, Periderm, which is thicker and tougher. A different secondary meristem, the vascular cambium, adds layers of vascular tissue (Secondary Xylem and Secondary Phloem). Wood is secondary xylem that accumulates over the years.

Secondary Xylem and Phloem Develop from Vascular Cambium Vascular Cambium is one or two cells thick. It is a persistent meristematic tissue, giving rise to secondary vascular tissues, resulting in a growth in diameter of stems. The Vascular Cambium divides and grows in both directions. Outside of the meristem, secondary phloem forms and within the meristem, secondary xylem forms. Formation of the Vascular Cambium

This figure illustrates how secondary xylem and secondary phloem are formed. The vascular cambium produces more xylem cells than phloem. Drawing of the divisions of vascular cambium cells through a growing season. C = Vascular Cambium Cells. X = Xylem Cells. P= Phloem Cells. Notice that more xylem cells are produced compared to phloem cells.

Wood is Composed of Secondary Xylem Over the years, a woody stem gets thicker and thicker as its vascular cambium produces layer upon layer of secondary xylem. The cells of the secondary xylem have thick walls rich in lignin, giving wood its characteristic hardness and strength. Wood is composed of 3 planes of view: tangential, radial and transverse. Structure of Red Oak Wood Axial System – Vessel members transport materials longitudinally. Ray System – Ray parenchyma and ray tracheids transport radially.

Drawing of a 4-year old woody stem showing the growth increments as annual rings. Annual rings are concentric rings of cells in the secondary xylem. In temperate climates (like us), one annual ring forms each growing season. You can roughly age a tree by counting the rings. In tropical climates, trees often have irregular growth rings because growth occurs all year. Redwood and bristlecone pines (very old trees) can be 1000s of years old and can be aged by rings. Dendrochronology is the study of tree rings. If a tree has a thick ring, it had a lot of growth during that year.

Section of 3-year old Basswood Stem Made up of the first new xylem cells to develop Earlywood cells (Spring Wood) are usually larger in diameter and thinner walled than those produced later in summer (Latewood = Summer Wood).

Secondary Xylem is made of heartwood and sapwood. - Heartwood no longer functions. It has resin which blocks transport. Resin is a viscose, sticky fluid of plant origin, used by wood plants to repel wood-burrowing insect pests and resin also prevents rotting. However, some trees have heartwood, which rot away while the tree still lives (Example: Beech). - Sapwood does function and it transports water & nutrients from the roots. Gymnosperm wood (Example: Pine) differs from Angiosperm wood (Example: Oak). Gymnosperm is much simpler. Some gymnosperm wood has resin ducts, which are secretory structures. Turpentine is made from resin. When resin flows to the outside of the tree it is called sap. When sap hardens, it is called rosin. Fossilized rosin is amber, which is used in jewelry.

Bark is Composed of Secondary Phloem & Periderm Bark is a result of secondary growth. Everything external to the vascular cambium is bark (Secondary Phloem and Periderm). External to the Vascular Cambium, the external tissues do not accumulate over the years like xylem. Instead, they are sloughed off at about the same rate they are produced. This is evident in the cracked, peeling bark of many tree trunks. Therefore, secondary phloem doesn’t develop into annual rings like secondary xylem. Trees can be killed by girdling. If you cut a ring around the tree into the bark, you will destroy the secondary phloem. The roots will not obtain the food from the leaves and the roots die and the whole tree dies. This is called girdling. This is a forestry technique used to remove unwanted trees and to create snags (standing dead trees) for wildlife use.

Periderm - Periderm is a secondary tissue, belonging to the dermal tissue system. It is formed from the secondary meristem named the Cork Cambium. Periderm replaces the primary tissue, the epidermis. Young trees have little periderm, while older trees have much periderm. - The periderm consists of cork (phellem), cork cambium (phellogen) and phelloderm. Cork, which is the outer layer, aids in protection. - Initially, outer cortex cells (parenchyma) form the cork cambium. Then, the cork cambium (secondary meristem) gives rise to cork and phelloderm cells to form the periderm. Formation of Periderm

Lenticel A lenticel is a structure of the bark that permits the passage of gas inward and outward of the plant. Lenticels are specialized regions of the periderm consisting of loosely packaged parenchyma cells. They are living cells that require oxygen gas. Lenticels are present in young branches of woody plants.

Buds Are Compressed Branches Waiting to Elongate Drawing of a Walnut Twig showing 3 years-worth of growth. Leaf scars and buds from Walnut (Left) and Catalpa (Right) trees.

Palm, a monocot, is unbranched and lacks true secondary growth. Most monocots lack a vascular cambium and therefore do not form secondary xylem and secondary phloem….. but some monocots have secondary growth. Palm, a monocot, is unbranched and lacks true secondary growth. Monocots such as tree lilies (such as yucca) and The Joshua tree have branched stems, a cambium and true secondary growth.

Stem Modifications For Special Functions Rhizome – an elongated underground horizontal stem. Rhizomes are often near the surface. Tuber – Enlarged terminal portion of underground rhizomes. Tubers are much-enlarged, short, fleshy underground stems. Example: The potato you eat. Young Potato Plant Strawberry Stolon – A stem that grows horizontally along the ground surface. Stolons are often call runners as they are above ground horizontal stems. Stolons help a plant spread. Examples: Strawberry and Bermuda Grass.

Stem (Shoot) Structures Modified for Storage of Food Corm – A short, solid, thickened, vertical, enlarged underground stem in which food is stored. Examples: Crocus, Gladiolas, and some Iris plants. Bulb – A short, flattened, or disk-shaped underground stem, with many fleshy-scale leaves filled with stored food. Food is stored in specialized, flesh leaves. Examples: onion, lilies, hyacinths, and tulips.

Modified Stems Cladophyll – A stem or branch resembling a leaf. Cladophyll are flattened photosynthetic stems that function as leaves and resemble leaves. Thorn – A sharply pointed woody structure; a modified branch. Thorns are formed from axils of leaves. Example: Hawthorn Tree Tendril – Can either be a stem or leaf. Tendrils are slender coiled organs that aid in the support of stems. Example: Cucumber The Following are Not Modified Stems: Prickles – Modified cluster of epidermal hairs. Example: Rose Bush Spine – Modified leaf. Example: Cactus Needles.

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