sambstm.htm

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Epidermal Tissues

The first cellular layer a plant presents to its environment is the epidermis. Although only a single cell thick on many plant structures, the epidermis of stems is typically waterproofed by a waxy secretion called cutin. This forms the cuticle layer found on the outer aspect of many epidermal tissues, particularly those associated with plant structures that are subjected to dessicating forces. Some epidermal cells are specialized as guard cells of stomata, trichomes on stems and leaves, or as root hairs on roots. You can see some of these epidermis-derived structures later in the image gallery.

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Cortical Parenchyma Cells

Cells located below the epidermis make up a region called the cortical region. Cortical regions can be extensive or limited depending on the plant structure, type of plant, and age. Cortical cells are mostly parenchyma cells but these can be interspersed with other cell types. The cortical cells of young stems are typically photosynthetic, containing chloroplasts. In other stems these cells may accumulate plastids like amyloplasts or chromoplasts. Parenchyma cells are capable of mitosis and therefore can be called upon to promote growth and production of more specialized cells. The thin-walled flattened cells just below the epidermal tissues may be producing new cell layers.. To the right of these are photosynthetic parenchyma cells with chloroplasts.

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Collenchyma Cells

Watch for collenchyma supportive tissues in stems, petioles, and other plant structures that require flexible support during growth. Thickened regions of primary cell walls of these cells provide plasticity and strength. Thinner areas enable diffusion for water, nutrients, and wastes. Think of a green stem or flower stalk. These structures elongate fast and remain supportive, until you cut or deprive them of water. Then they wilt! This is a sign the supportive tissues are living collenchyma and/or parenchyma types, dependent on turgor pressures. Hardened, woody stems deprived of water remain upright and rigid due to the lignified secondary walls of sclerenchyma tissues.

One more page for review!

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Review of Terms Relating to Stem Structure

In a couple of the units that follow you will be looking at actual microscopic images of monocot, dicot, and woody stems at different magnifications. It is valuable for you now to familiarize yourself with terms applied to stem regions since they will be used in these units.

Remember apical meristems? All mature stem tissues are derived from these. The first tissues to form are the three embryonic tissues, protoderm, procambium, and ground meristem. These form the cells that differentiate into the mature tissues of a stem. Protoderm forms the epidermal tissues and these are always outermost. The ground meristem forms the cortical and pith tissues. Between cortical and pith tissues, procambium forms vascular tissues, the xylem and phloem. In a stem that exhibits only primary growth, xylem and phloem will always be found in vascular bundles. In stems with secondary growth, xylem and phloem are found to either side of a secondary meristem called the vascular cambium.

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Stem Overview

Most stems need protection from dessication. Subsequently, in stem slides you may see a specialized cuticle layer on epidermal cells. In stems with secondary growth, a secondary meristem, the cork cambium forms bark. Herbaceous or young stems can be photosynthetic and therefore contain cells with chloroplasts, particularly in outer cell layers. Stems also need cells that can engage in mitosis to produce new cells for growth. Keeping these points in mind, let's use a 400x view of a Sambuchus stem to present some basic stem anatomy.

Notice variations in cell walls here and remember parenchyma cells only contain primary cell walls. This allows mitotic divisions and more complex metabolic functions. You can therefore expect parenchyma cells to be abundant in stem tissues.

Now find some cells with irregular cell wall thickenings in this view. These are collenchyma tissues providing support and plasticity during rapid growth. Flexibility or plasticity of tissues enables fast-growing stems to sway in the wind or bend under weight of rain or dew. Think how fast actively growing "green" shoots can elongate!

Lets locate some of these cells in stem tissues. Roll your mouse over each feature to the right to locate these in the image.

Page 2 of 5 Epidermal Tissues The first cellular layer a plant presents to its environment is the epidermis. Although only a single cell thick on many plant structures, the epidermis of stems is typically waterproofed by a waxy secretion called cutin. This forms the cuticle layer found on the outer aspect of many epidermal tissues, particularly those associated with plant structures that are subjected to dessicating forces. Some epidermal cells are specialized as guard cells of stomata, trichomes on stems and leaves, or as root hairs on roots. You can see some of these epidermis-derived structures later in the image gallery. Page 3 of 5 Cortical Parenchyma Cells Cells located below the epidermis make up a region called the cortical region. Cortical regions can be extensive or limited depending on the plant structure, type of plant, and age. Cortical cells are mostly parenchyma cells but these can be interspersed with other cell types. The cortical cells of young stems are typically photosynthetic, containing chloroplasts. In other stems these cells may accumulate plastids like amyloplasts or chromoplasts. Parenchyma cells are capable of mitosis and therefore can be called upon to promote growth and production of more specialized cells. The thin-walled flattened cells just below the epidermal tissues may be producing new cell layers.. To the right of these are photosynthetic parenchyma cells with chloroplasts. Page 4 of 5 Collenchyma Cells Watch for collenchyma supportive tissues in stems, petioles, and other plant structures that require flexible support during growth. Thickened regions of primary cell walls of these cells provide plasticity and strength. Thinner areas enable diffusion for water, nutrients, and wastes. Think of a green stem or flower stalk. These structures elongate fast and remain supportive, until you cut or deprive them of water. Then they wilt! This is a sign the supportive tissues are living collenchyma and/or parenchyma types, dependent on turgor pressures. Hardened, woody stems deprived of water remain upright and rigid due to the lignified secondary walls of sclerenchyma tissues. One more page for review! Page 5 of 5 Review of Terms Relating to Stem Structure In a couple of the units that follow you will be looking at actual microscopic images of monocot, dicot, and woody stems at different magnifications. It is valuable for you now to familiarize yourself with terms applied to stem regions since they will be used in these units. Remember apical meristems? All mature stem tissues are derived from these. The first tissues to form are the three embryonic tissues, protoderm, procambium, and ground meristem. These form the cells that differentiate into the mature tissues of a stem. Protoderm forms the epidermal tissues and these are always outermost. The ground meristem forms the cortical and pith tissues. Between cortical and pith tissues, procambium forms vascular tissues, the xylem and phloem. In a stem that exhibits only primary growth, xylem and phloem will always be found in vascular bundles. In stems with secondary growth, xylem and phloem are found to either side of a secondary meristem called the vascular cambium. Forward to the next topic. Back to the module outline.
Page 1 of 5 Stem Overview Most stems need protection from dessication. Subsequently, in stem slides you may see a specialized cuticle layer on epidermal cells. In stems with secondary growth, a secondary meristem, the cork cambium forms bark. Herbaceous or young stems can be photosynthetic and therefore contain cells with chloroplasts, particularly in outer cell layers. Stems also need cells that can engage in mitosis to produce new cells for growth. Keeping these points in mind, let's use a 400x view of a Sambuchus stem to present some basic stem anatomy. Notice variations in cell walls here and remember parenchyma cells only contain primary cell walls. This allows mitotic divisions and more complex metabolic functions. You can therefore expect parenchyma cells to be abundant in stem tissues. Now find some cells with irregular cell wall thickenings in this view. These are collenchyma tissues providing support and plasticity during rapid growth. Flexibility or plasticity of tissues enables fast-growing stems to sway in the wind or bend under weight of rain or dew. Think how fast actively growing "green" shoots can elongate! Lets locate some of these cells in stem tissues. Roll your mouse over each feature to the right to locate these in the image. .