Lecture 3 Plant Anatomy

Collenchyma Collenchyma consists of cells that are living and slightly elongated. Collenchyma is plastic, particularly when young, and it stretches irreversibly with the growth of the organs in which it is found. Collenchyma functions as a supporting tissue in young growing organs and even in matur...

Collenchyma Collenchyma consists of cells that are living and slightly elongated. Collenchyma is plastic, particularly when young, and it stretches irreversibly with the growth of the organs in which it is found. Collenchyma functions as a supporting tissue in young growing organs and even in mature organs in herbaceous plants. The walls of collenchyma can become lignified thus forming sclerenchyma. Distribution of collenchyma in the plant body Collenchyma may occur in stems, leaves, floral parts and in roots. In all cases collenchyma exists in the form of groups of cells and not solitary cells Collenchyma is absent from the stems and leaves of monocotyledons where sclerenchyma develops at an early age Structure and contents of collenchyma cells Collenchyma like parenchyma contains chloroplasts. The walls of collenchyma are composed of alternating layers with different proportions of cellulose and pectin. But in general, the cells are rich in pectin.  Types of collenchyma 1- Angular collenchyma in which the thickening of the cell wall is longitudinal along the angles of the cell. 2- Lamellar collenchyma in which the thickenings are mainly on the tangential walls. 3- Annular collenchyma in which the thickening of the cell wall is almost homogenous and therefore the cell lumens are circular  Suberized cell wall Lamellar collenchyma Remember Suberized cells are largely impermeable to water and provide protective layer preventing water loss. Suberin is a specialized cell wall, which is deposited in dermal tissues and above-ground periderm (i.e., bark).  Sclerenchyma Sclerenchyma is composed of cells with thick secondary walls that are usually lignified. Sclerenchyma cells are elastic (retain their original size and shape after being stretched) The function of sclerenchyma is support and sometimes protection. Mature sclerenchyma is composed of dead cells with extremely thick cell walls (secondary walls) that make up to 90% of the whole cell volume. Two groups of sclerenchyma cells exist: fibers and sclereids. Fibers are long cells while sclereids are short. Fibers and sclereids occur in different parts of the plant body. Fibers develop from meristems while sclereids develop from parenchyma that becomes lignified (origin ‫)منشأها‬. Example Phloem fibers Osteosclereids Astrosclereids  Sclereids Sclereids are usually dead cells, occur as hard masses of cells within soft parenchyma tissues. Types of Sclereids 1- Osteosclereids: These are bone shaped cells with enlarged or branched ends. They are found in seeds, coats and in the leaves of some dicotyledons. 2- Astrosclereids: They are very branched and usually star-shaped. Such sclereids are found in leaves and stems of Hydrophytes. Osteosclereids Astrosclereids