Plant cell the major classes of cells and tissues of roots, stems, leaves, flowers, and reproductive structures, each of which may be composed of several cell types. Parenchyma cells are living cells that have functions ranging from storage and support to photosynthesis (mesophyll cells) and phloem loading (transfer cells). Apart from the xylem and phloem in their vascular bundles, leaves are composed mainly of parenchyma cells. Some parenchyma cells, as in the epidermis, are specialized for light penetration and focusing or regulation of gas exchange, but others are among the least specialized cells in plant tissue, and may remain totipotent, capable of
Smear layer 1975 McComb and Smith first described the smear layer. They observed an amorphous layer of debris, with an irregular and granular surface, on instrumented dentinal walls using a scanning electron microscope (SEM). This smear layer was composed of dentin, pulp and bacterial remnants. The authors stated that “most standard instrumentation techniques produced a canal wall that was smeared and packed with debris.” In 1984 Pashely described the smear layer as being composed of two phases; an organic phase, composed of collagen residues and glycosaminoglycans from extracellular matrix of pulp cells, which acts as a matrix for an inorganic phase. This