We will discuss only the details specific to stems. The vascular cambium is located between the primary xylem and primary phloem within the vascular bundle.
Recall that xylem is located toward the interior and phloem toward the exterior of the bundle. The cells of the vascular cambium divide and form secondary xylem tracheids and vessel elements to the inside, and secondary phloem sieve elements and companion cells to the outside. The cells of the secondary xylem contain lignin , the primary component of wood, which provides hardiness and strength.
The xylem together with the pith form the wood of a woody stem. In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells , which contain a waxy substance that can repel water. The phloem together with the cork cells form the bark , which protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm , which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm.
The periderm substitutes for the epidermis in mature plants. The combined actions of the vascular and cork cambia together result in secondary growth, or widening of the plant stem. These structures are illustrated below:. In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends from the vascular cambium to the epidermis.
A new layer of xylem and phloem are added each year during the growing season. The interior xylem layers eventually die and fill with resin, functioning only in structural support. The interior, nonfunctional xylem is called heartwood. The newer, functional xylem is called sapwood. The exterior layers of phloem eventually become crushed against the cork cambium and are broken down. Thus a mature tree contains many interior layers of older, nonfunctional xylem deep within the stem, but only a small amount of older phloem.
The layers of tissues within a mature tree trunk. The activity of the vascular cambium results in annual growth rings.
During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened. This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids.
It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem shown below.
An examination of the number of annual rings and their nature such as their size and cell wall thickness can reveal the age of the tree and the prevailing climatic conditions during each season.
The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring for each year of growth. Start your free trial today and get unlimited access to America's largest dictionary, with:. Accessed 12 Nov. Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free! Log in Sign Up. Save Word. Definition of indeterminate growth. Love words? Start your free trial today and get unlimited access to America's largest dictionary, with: More than , words that aren't in our free dictionary Expanded definitions, etymologies, and usage notes Advanced search features Ad free!
Join Our Free Trial Now! The term is used to reference animals that grow rapidly when young, and continue to grow after reaching adulthood, albeit at a much slower pace. For example, fish, reptiles, and many mollusks are known for their indeterminate growth characteristics.
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