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Peripheral meristems
Peripheral meristems
Peripheral meristems, or secondary meristems, are areas of actively dividing cells in the periphery of plants
A peripheral meristem produces secondary growth: cells created through mitosis to increase the girth of the plant
Secondary growth comes from a ring of cells called cambiums (e.g. vascular cambium, cork cambium)
In some plants, the cambiums are found in the periphery of the stems, as well as the periphery of roots
The vascular cambium produces wood and secondary phloem
The cork cambium produces bark
Secondary Growth
Secondary Growth
By definition, secondary growth is cell division from a meristem that forms after the primary meristem.
For example, the apical meristem produces primary growth, and these cells/tissues are left to mature. Some of these cells can form into secondary meristems to produce secondary growth.
Most examples of secondary growth come from meristems that are found on the edges of the plant; therefore, they are called peripheral or lateral meristems
The vascular cambium, located underneath the bark of the trunk, is a common example of a peripheral secondary meristem found in woody plants.
Secondary (peripheral) growth increases the girth of the plant's shoots or roots, and they are associated with being woody.
Secondary growth is primarily found in woody plant stems and roots, and it is the hallmark of a "true tree."
There are examples of anomalous secondary growth in some monocots and non-seed plants, but these are not common in the modern flora, and sometimes not peripheral.
Types of secondary (peripheral) meristems
Types of secondary (peripheral) meristems
There are two common types of peripheral secondary meristems found in woody plants:
Vascular cambium: Creates secondary vascular tissues for woody plants, such as wood
Cork cambium: Creates bark for woody plants
Vascular cambium
Vascular cambium
The vascular cambium is bifacial, or 2-faced. It is a layer of actively dividing cells that creates both secondary xylem and secondary phloem for a woody plant.
Also called wood; secondary xylem is, by definition, the type of xylem created by a vascular cambium
Secondary xylem is produced toward the inside (centrifugal) of the vascular cambium in the stem
Xylem cells (e.g. vessel elements, tracheids) are produced to move water longitudinally from roots to leaves
Xylem ray cells are produced to move water laterally from outside to inside
Also called inner bark; secondary phloem is, by definition, phloem created by a vascular cambium
Secondary phloem is produced toward the outside (centripetal) of the vascular cambium in the stem. It is usually sandwiched between the bark and the wood
Phloem cells (e.g. sieve tubes, sieve cells) are created to move sugars longitudinally, from leaves to actively growing or storage areas
Phloem ray cells are produced to move solutes from outside to inside
Above: Animated GIF of woody growth at the microscopic level, showing how the vascular cambium produces cells.
Above: Cross-section of wood. The regularly aligned cells at the top of the image (#2-3) are secondary xylem (or wood) cells. The cells at the bottom are secondary phloem, with #4 sieve tubes, and #6 phloem fibers. The cells at the very bottom of the image (#5) are periderm cells or bark.
Above: Cross-section of a 3-year old Basswood (Tilia) stem
Heartwood & Sapwood
Heartwood & Sapwood
These are terms used in the lumber industry to refer to different types of wood within a trunk that may have different coloration patterns depending on the tree species
The secondary xylem cells that are closest to the vascular cambium are the youngest and function in moving water and minerals. This area is called the sapwood, and it is sometimes light-colored in certain species
As trees increase in girth, they leave behind older cells at the interior of the stem. These cells become clogged and sometimes discolor. This is called the heartwood, and this can be the valuable wood that gives species like walnut, mahogany, and cherry their characteristic color.
Cork cambium
Cork cambium
The cork cambium, also called the phellogen, creates bark tissues for a woody plant, which replaces the epidermis in woody plants. This actively growing zone produces phelloderm cells to the inside and phellem cells to the outside.
Phelloderm is the bark tissue produced toward the inside of the cork cambium. It is composed of living parenchyma cells and is sometimes used for storage.
Phellem, also called cork, is bark tissue produced toward the outside of the cork cambium. It is dead at maturity, and it possesses air pockets as a protective tissue
Evolution of wood
Evolution of wood
Plants with secondary growth form a clade called the lignophytes ("lignified plants")
Some of the first plants to exhibit secondary xylem were ancestral euphyllophytes and the extinct, spore-bearing group called the progymnosperms.
The ancestors of all seed plants (including flowering plants) are thought to be the progymnosperms with secondary xylem.
Some groups, like the monocots, lost this woody ability during evolution
Secondary xylem has also independently evolved in clubmosses (e,g., Lepidodendrales), horsetails (e.g., Calamitaceae), and possibly other groups.
Additional Resources
Additional Resources
5 ingenious things trees do that human designers can learn from (The Conversation 3Aug2025)
Lessons from the heartwood: Resilience in regenerating redwoods (Phys.org 26Jun2025)
└Sillett et al. (2025) Improved allometry and heartwood development of Sequoia sempervirens in secondary forests
Could trees know when the summer solstice is? (The Conversation 19Jun2025)
Scientists discover entirely new wood type that could be highly efficient at carbon storage (Phys.org 31July2024)
└Lyczakowski & Wightman (2024) Convergent and adaptive evolution drove change of secondary cell wall ultrastructure in extant lineages of seed plants
Evolutionary history shapes variation of wood density (Phys.org 13 May 2024)
└Li et al. (2024) Evolutionary history shapes variation of wood density of tree species across the world
Pokeweed's tree-like cousin (In Defense of Plants 2018): independent evolution of secondary growth (anomalous secondary thickening), rather than true wood
Trees' internal water pipes predict which species survive drought (Princeton Univ 2016)
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