Secondary Growth
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
By definition, secondary growth is mitotic growth from a peripheral meristem (e.g. vascular cambium)
Contrasted with primary growth which comes from an apical meristem
Secondary growth increases the girth of the plant's shoots or roots
Secondary growth is found only in woody plant stems and roots
This feature is the hallmark of a "true tree"
Types of secondary meristems (or peripheral meristems)
There are two common types of peripheral meristems:
Vascular cambium: Creates secondary vascular tissues for woody plants, such as wood
Cork cambium: Creates bark for woody plants
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: 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
Cork cambium
The cork cambium, also called phellogen, creates bark tissues for a woody plant, which replaces the epidermis in woody plants
Phelloderm are bark tissues produced toward the inside of cork cambium
Phelloderm is composed of living parenchyma cells
Phellem, also called cork, are bark tissues produced toward the outside of the cork cambium
Phellem is dead at maturity
Cells have air pockets as a protective tissue
Evolution of wood
Plants with secondary growth form a clade called the lignophytes.
Some of the first groups to exhibit secondary xylem were the extinct, spore-bearing group called the progymnosperms.
All seed plants (including flowering plants) came from a progymnosperm ancestor with secondary xylem.
Some groups, like the monocots, lost this ability during evolution
Secondary xylem has also independently evolved in clubmosses (e,g. Lepidodendrales), horsetails (e.g. Calamitaceae), and possibly other groups.
Additional Resources
Evolutionary history shapes variation of wood density (Phys.org 13May2024)
└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)