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Parenchyma cells, the progenitor of all other cell types, are composed of thin walled, globular, more or less undifferentiated cells. Parenchyma cells comprise many soft tissues of plants (e.g., pith, cortex, leaf mesophyll, etc.). These cells also compose the horizontal rays in wood. Parenchyma cells retain the ability to divide throughout their lives, so they are important in vegetative regeneration and wound healing. For example, roots growing from a stem cutting are created and differentiate from parenchyma cells that are scattered throughout the stem and spring into action when cued by hormonal changes that a new structure is needed. Most of the "work" of plants (e.g., photosynthesis, carbohydrate storage, metabolism, secretion, and biosynthesis) occurs in parenchyma cells. As parenchyma is incorporated into vascular tissue (rays in wood for example), it also helps in the movement of water and solutes throughout the plant body. Because parenchyma tissue is composed of only one cell type, parenchyma is called a simple tissue.
SCLERENCHYMA
Sclerenchyma cells have thick, lignified secondary walls, lack cell contents at maturity, and occur throughout all plant tissues. These features make sclerenchyma tissues hard, rigid, and somewhat brittle. Sclerenchyma cells can occur as aggregates within ground tissue (sclereids or stone cells or as elongated fibers. In this context, sclerenchyma provides mechanical strength to stems (fibers in hemp and flax) and reproductive structures (the texture in pear flesh, the stony shells of nuts and cherry pits).
(Note that you can distinguish collenchyma cells from sclerenchyma cells because of the chemical composition of their cell walls. Different biological stains are attracted to either cellulose or lignin. Consequently, in the most common stain system, cellulose stains blue or green and lignin stains reddish or pink.)
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