Hawaiian
Ethnobotany-Anatomy-18 
Hawaiian
Ethnobotany-Anatomy-18 
Secondary Growth-1
Growth in width comes from cells which only divide parallel to the surface and produce derivatives laterally. Cells produced towards the inside become Secondary Xylem. Those produced towards the outside become Secondary Phloem.
This Lateral Meristem is the Vascular Cambium.
Wood is Secondary Xylem. It contains thick-walled conducting cells (Vessel Members) and Fibers. Both add strength to the stem or root. The differences in wood characteristics ( hard vs soft ) depend on the proportions of these cells and their wall traits. Wood is typically Lignified & this further increases its strength.
I will use Coleus to illustrate Secondary Growth in Stems.
Coleus
has a square stem with large Vascular Bundles in each corner. |
Each
Vascular Bundle contains Xylem, Phloem and Fibers . |
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| Same as above viewed with Polarized Light | |
Ground Tissue occupies most of the area between the large vascular bundles. Some small vascular bundles are present, however. |
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Cell Divisions continue in a layer which separates the Xylem & Phloem. This produces new "Secondary" Xylem that expands the size of the bundles in each corner. |
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Cell Divisions spread to the Ground Tissue between each
Vascular Bundle. This also produces new |
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The Lateral Meristem
responsible The initial extent of Secondary Growth can be seen in this overall section. This process continues for centuries in woody plants like Koa. Far more Secondary Xylem is produced than Secondary Phloem. |
|
 Corner of a Coleus Stem that has produced Secondary Xylem |
 Region between Corner Bundles in a Coleus Stem with some Secondary Growth. This region of the Vascular Cambium only produces Secondary Xylem. |
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| Coleus Stem with Primary Growth | Coleus Stem after a small amount of Secondary Growth |
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| Stem sections similar to those of Coleus viewed with normal illumination (left) and polarized illumination (right). | |
