Tissues
Plants are a composite of cells organized into tissues. Every cell within these tissues has a unique size and shape and is surrounded by a wall composed of a complex carbohydrate called cellulose. Plant cells are attached to each other by a gluelike substance, pectin, that cements them together.
All plant tissues originate in meristems, which are unique tissues of the plant body. They are the sites of new cell production and of the genetic events necessary for cellular specialization. Meristems can be categorized by their locations. Apical meristems are composed of groups of dividing cells at the tips of shoots (branches) and roots. When meristematic cells produced by apical meristems begin elongating, they are classified as primary meristems. There are three types of primary meristems: protoderm, ground meristem, and procambium. As primary meristem cells stop dividing and begin differentiating, their classification changes to primary tissues. There are four primary tissues: the epidermis derived from protoderm, the ground tissues (parenchyma, sclerenchyma, and collenchyma) derived from ground meristem, and two types of vascular tissue—xylem and phloem—derived from procambium. Primary tissues have specific positions in the plant body and specific functions.
As the organs of the plant body age, the stems and roots often grow wider.
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