Plant Root Development, Structure & Function

 

I.              Root development and structure

                A.            Root cap - protects the tip of the root

                B.            Apical meristem - gives rise to meristematic tissue

                                1.             Quiescent center - frequency of mitoses is low

                                2.             Proximal meristem

                                                a)             Protoderm - produces epidermis

                                                b)             Ground meristem - produces ground tissue (cortex and endodermis)

                                                c)             Procambium - produces vascular tissue (vascular cylinder referred to as the stele)

                C.            Developmental zones of the root

                                1.             Zone of cell division (meristematic zone) - tip of root

                                                a)             Apical meristem & primary meristematic tissues

                                2.             Zone of cell elongation (above division zone)

                                                a)             Cells grow in length & diameter - little division or differentiation

                                3.             Zone of cell differentiation (uppermost - kinda where root hairs are visible)

                                                a)             Secondary wall formation

                                                b)             Formation of root hairs and endodermis

                D.            Primary root cross-section

                                1.             Dicot

                                                a)             Epidermis - outermost layer - no cuticle - produces hairs

                                                b)             Cortex - middle tissue between epidermis and vascular cylinder - undifferentiated parenchyma cells - major region of nutrient absorption

                                                c)             Endodermis - encases the vascular cylinder (stele) - seals off stele from cortex

                                                                1)            Casparian strip - waxy thickening in cell walls of endodermal cells - contains suberin and waxes - keeps water in

                                                d)             Stele

                                                                1)            Everything inside endodermis

                                                                2)            Xylem occurs in radiating arms in center (diarch, triarch, etc.)

                                                                3)            Phloem occurs between xylem arms

                                                                4)            Pericycle forms a layer of parenchyma cells just to the inside of the endodermis

                                2.             Monocot - similar to dicot root, but with pith in center of the stele

                                                a)             Monocot stele is different because it doesn't really have "arms" and because its vascular tissue is in a polyarch arrangement

                E.             Secondary tissue of roots - laid down similarly to that of shoots

                                1.             Vascular:  Secondary xylem to the inside; secondary phloem to the outside

                                2.             Cork:  Phellem to the outside and phelloderm to the inside (root bark consists of phellem and phelloderm)

II.            Root function

                A.            Anchorage - holds plants in place

                B.            Storage - reservoir of carbohydrates and other organics

                C.            Conduction - passageway for nutrients

                D.            Absorption - take up water and nutrients