Cells

Plant tissues possess discrete cells, a construction conferring strength, tolerance of localized injury, division of labor, and (via intercellular air spaces) gas exchange. The cell is composed of a protoplast (nucleus plus cytoplasm) and a cell wall. Three organelle types contain genes - the nucleus, plastids, and mitochondria, each bounded by a double membrane. Plastids (e.g., chloroplasts) and mitochondria share many basic structural and functional features, including an ancient bacterial origin. Cellular components without deoxyribonucleic acid, bounded by single membranes, include vacuoles, endoplasmic reticulum, Golgi bodies, plasma membrane, and peroxisomes. The rest of the cytoplasm is the cytosol, containing ribosomes and a cytoskeleton. The cell wall imposes the cell's shape and size and the adhesive middle lamella dictates the cell's position in relation to its neighbors. Cell shape, size, and position, plus cell wall chemistry, characterize differentiated tissues - which are briefly described, including meristems, epidermis, parenchyma, laticifers, collenchyma, sclerenchyma, xylem, phloem and cork.