Biology: Plant Organs, Tissues, and Cells: Part Four
Plant Tissues
Ground Tissue: Parenchyma
Parenchyma cells are a generalized plant cell type that remains alive at maturity. The cells make up the bulk of ground and vascular tissues. They tend to have relatively thin, flexible cell walls and perform many functions in the plant. Palisade parenchyma cells are elongated cells located in the leaves of many plants, just below the epidermal tissue. Spongy parenchyma cells occur below the one or two layers of palisade cells. Both of these types of parenchyma carry out photosynthesis and feature many chloroplasts. Ray parenchyma cells occur in wood rays, the structures that transport materials laterally within a woody stem. Parenchyma cells also occur within the xylem and phloem of vascular bundles. The largest parenchyma cells occur in the central pith region of the stem. Often, as in corn (Zea) stems, these cells are larger than the vascular bundles. In most commercially prepared slides, parenchyma cells stain green. This differential staining is caused by differences in cell wall composition.
Examples of parenchyma cells. Diagram of leaf structure.
Scanning electron micrograph of a leaf cross-section of Tradescantia.
Microscopic view of parenchyma cells. Note the relatively thin walls.
Ground Tissue: Collenchyma
Collenchyma cells are characterized by thickened cell walls that allow them to support the plant stem’s weight and to raise the stem above the ground. Collenchyma cells are alive at maturity. They often occur as part of bundles of vascular tissue (xylem and phloem). Many angular stemmed plants, such as Coleus and other mints, have collenchyma cells clustered at the angles of the stem, supporting and shaping the stem. In prepared slides, collenchyma cell walls retain a red stain.
Collenchyma. from the stem of Medicago, an angular stemmed plant in the mint family. Left image is stained, right image is not.
Ground Tissue: Sclerenchyma
Sclerenchyma cells have very thick walls, which allow them to physically support the soft stems of many plants. These cells often occur in masses, capping the vascular bundles of many plants. Thickenings in their secondary walls contain chemicals that cause them to stain red. Unlike collenchyma, however, sclerenchyma cells are dead at maturity. A common type of schlerenchyma cell is the fiber, which can be used in making linen and other cloth. Some sclerenchyma cells occur in the fruits of pears. These cells (sclereids or stone cells) give pears their gritty texture.
Sclereids from Basswood stem. Sclereids are the red cells with very thick walls.
Leaf vein and surrounding cells.
Now go on to the next part. 
| 
