magstmic.htm

Microscopy of a Magnolia Stem

100x Magnification

In a 100x view, characteristics of this dicot stem start to emerge. To the left of center are some groups of sclerenchyma fibers. These are supportive tissues associated with vascular bundles. The vascular bundles of a dicot stem are arranged in a broken ring. Primary meristems of the vascular bundles(fascicular cambium) join interfascicular cambium to form vascular cambium, a continuous ring of meristematic cells encircling the stem. The vascular cambium of woody dicots is the meristem that produces yearly phloem and xylem. Note the phloem between the vascular cambium and the fiber caps! Xylem formed each growth season is found in concentric rings that make up the secondary growth or yearly expansions of the wood. A single growth ring of xylem here indicates this is a one year old stem.

200x Magnification

In this 200x view, the characteristics of the cell types are a little clearer. Notice the thick secondary walls of the sclerenchyma fibers to the left. Immediately to the right of these are the phloem tissues produced by the meristem of the vascular bundle. Thick secondary cell walls of dead, empty-looking xylem cells on the right stand in sharp contrast to those of the phloem which are mostly parenchyma cell types. The vascular cambium consists of one or two lines of cells between the xylem and phloem. Interspersed among the xylem cells to the right are linear arrays of living parenchyma cells that function in radial transport within the stem. These cells form the vascular rays.

400x Magnification

The 400x view provides a more limited view of the vascular cambium area. The sclerenchyma fibers lie just outside this field to the left. Most of the tissue in this view is phloem and xylem. Look for nuclei in some of the darker cells to the left. These are companion cells of the phloem. To the right are the thick-walled xylem cells and radial rows of vascular ray parenchyma cells. Look closely at the boundary between xylem and phloem for one or two thin lines of flattened cells of the vascular cambium.

1000x Magnification

At 1000x magnification, the important vascular cambium region is hardly striking! Envision mitotic cell divisions in this layer producing new cells to either side. Cells to the left differentiate into new phloem or supportive cells of the phloem. Cells to the right differentiate into xylem. Xylem cells look empty because most die as their secondary walls thicken and accumulate lignin. A nucleus of a living, vascular ray parenchyma cell is visible to the right. Remember, xylem forms the wood that accumulates in stems of trees and shrubs. The small growth rings visible in wood represent yearly vascular cambium activity. Ages of old trees are determined by counting these rings. In addition, the thicknesses of rings provides insight into historic climatic fluctuations that influenced the yearly growth of the tree.

In this low magnification view of a Magnolia stem, a number of different tissue layers are visible. On the left, the epidermis forms the outer layer of this dicot stem. Trichomes and a thick cuticle layer are visible as components of this epidermis. Below the epidermis, parenchyma cells of the cortex are visible in a well-defined layer, indicating this is a young stem. Eventually, as xylem (wood) accumulates, cortical tissues will be fragmented and lost. The vascular cambium, a secondary meristem, is visible as a line separating denser stained cells to the left and more open cells to the right. Internal to the vascular cambium are the xylem vascular tissues and the innermost parenchyma cells of the pith.

	Microscopy of a Magnolia Stem 100x Magnification In a 100x view, characteristics of this dicot stem start to emerge. To the left of center are some groups of sclerenchyma fibers. These are supportive tissues associated with vascular bundles. The vascular bundles of a dicot stem are arranged in a broken ring. Primary meristems of the vascular bundles(fascicular cambium) join interfascicular cambium to form vascular cambium, a continuous ring of meristematic cells encircling the stem. The vascular cambium of woody dicots is the meristem that produces yearly phloem and xylem. Note the phloem between the vascular cambium and the fiber caps! Xylem formed each growth season is found in concentric rings that make up the secondary growth or yearly expansions of the wood. A single growth ring of xylem here indicates this is a one year old stem. 200x Magnification In this 200x view, the characteristics of the cell types are a little clearer. Notice the thick secondary walls of the sclerenchyma fibers to the left. Immediately to the right of these are the phloem tissues produced by the meristem of the vascular bundle. Thick secondary cell walls of dead, empty-looking xylem cells on the right stand in sharp contrast to those of the phloem which are mostly parenchyma cell types. The vascular cambium consists of one or two lines of cells between the xylem and phloem. Interspersed among the xylem cells to the right are linear arrays of living parenchyma cells that function in radial transport within the stem. These cells form the vascular rays. 400x Magnification The 400x view provides a more limited view of the vascular cambium area. The sclerenchyma fibers lie just outside this field to the left. Most of the tissue in this view is phloem and xylem. Look for nuclei in some of the darker cells to the left. These are companion cells of the phloem. To the right are the thick-walled xylem cells and radial rows of vascular ray parenchyma cells. Look closely at the boundary between xylem and phloem for one or two thin lines of flattened cells of the vascular cambium. 1000x Magnification At 1000x magnification, the important vascular cambium region is hardly striking! Envision mitotic cell divisions in this layer producing new cells to either side. Cells to the left differentiate into new phloem or supportive cells of the phloem. Cells to the right differentiate into xylem. Xylem cells look empty because most die as their secondary walls thicken and accumulate lignin. A nucleus of a living, vascular ray parenchyma cell is visible to the right. Remember, xylem forms the wood that accumulates in stems of trees and shrubs. The small growth rings visible in wood represent yearly vascular cambium activity. Ages of old trees are determined by counting these rings. In addition, the thicknesses of rings provides insight into historic climatic fluctuations that influenced the yearly growth of the tree. In this low magnification view of a Magnolia stem, a number of different tissue layers are visible. On the left, the epidermis forms the outer layer of this dicot stem. Trichomes and a thick cuticle layer are visible as components of this epidermis. Below the epidermis, parenchyma cells of the cortex are visible in a well-defined layer, indicating this is a young stem. Eventually, as xylem (wood) accumulates, cortical tissues will be fragmented and lost. The vascular cambium, a secondary meristem, is visible as a line separating denser stained cells to the left and more open cells to the right. Internal to the vascular cambium are the xylem vascular tissues and the innermost parenchyma cells of the pith.



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