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Parenchyma Cells and Tissues
This image of parenchyma cells was used earlier to emphasize
vacuoles. The term "parenchyma" was also introduced in the plastid
unit. Keep in mind parenchyma cells possess thin primary cell walls. Large
vacuoles can make these cells appear empty when stained but they are hardly
non-functional in the living plant tissues. Note the abundant intercellular
spaces that are typically present. These are sites of apoplastic transport.
Parenchyma are the most abundant cell type. You should expect
to find these in all plant parts. In leaves and stems parenchyma are the only
photosynthetic cells. When filled with chloroplasts, they are called chlorenchyma.
Parenchyma cells are better suited for photosynthetic activity because their
thin primary cell walls allow more efficient passage of light, water, gases,
and metabolites. Remember, as walls of cells become thicker, transport processes
are affected!
You might also recall parenchyma are the only cells that can
engage in mitotic divisions. Subsequently, they are the only type of cell
found in apical meristems. Some of these are shown in various stages of division
in the mitosis unit.
In roots or stems parenchyma cells can be found as storage
cells filled with starch. Some of these were shown in the plastid module.
Watch for these in the Ranunculus root microscopy unit!
Specialized parenchyma is found in aquatic plants that require
flotation for their leaves. The open, air-filled parenchyma tissue in these
leaves is called aerenchyma.
Roll your mouse over the triangular intercellular
space next to the round nucleus.
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Cell Types
This is a partial view of a plant stem that contains a number
of tissues. Three basic types of plant cells are identifiable here. You
can apply your understanding of parenchyma cells now by looking for these
in the image. See if you can correctly pick out all parenchyma cells.
To check, roll your mouse over the parenchyma button.
Now,
roll over this second button for information about
collenchyma!
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Collenchyma Cells and Tissues
In contrast to parenchyma, collenchyma cells possess thickened
primary cell walls. These can be uniform but are typically uneven, a characteristic
you might remember as useful for identification. Like parenchyma, collenchyma
cells are also alive at maturity. The irregular thickened primary cell walls
of collenchyma cells provide support and plasticity in stems and leaves
during rapid growth.
Consider support needs of any young, green stem that has
produced many leaves. Cells derived from bud meristems lengthen, enlarge,
and differentiate to form the stem tissues during rapid growth. The stem,
now near maturity supports considerable weight due to the leaves. Each leaf
is also suspended on a flexible petiole. Wind and rain subject this stem
and its leaves to a variety of twisting and bending forces that can be substantial.
Collenchyma provides the flexible support and resistance necessary to counter
these forces.
Now, roll your mouse around
the collenchyma tissues!
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Great!
This is a view of outer tissues in a stem. Collenchyma in the upper right
corner is supportive tissue located below the epidermis just beyond this view.
The parenchyma cells immediately below the collenchyma make up the stem cortex.
Some phloem and xylem vascular tissues are visible in the lower left image
quarter. Contrast the parenchyma cells of the phloem with the thick-walled
xylem cells. Xylem cells possess uniformly thickened secondary cell walls.
We will learn about these vascular tissues soon.
Forward to the next topic.
Back to the module outline.
