Biological Foundations 112, Lecture 8

Root Anatomy and Morphology

Chapter 31, pages 668-691
Plant Anatomy-1   0:00-  5:19
Plant Anatomy-3 19:43-30:30


I.   Root Morphology

     A.   Function of roots

            1.   Anchorage

            2.   Absorption - root/shoot ratio must be in balance

            3.   Conduction

            4.   Storage

     B.   Root Systems

            1.   Fibrous roots - characteristic of grasses and monocots

            2.   Taproot - characteristic of dicots (carrot and turnip)

     C.   Development of root system

            1.   Radicle - embryonic root

            2.   Primary root - the first root formed from the radicle

            3.   Secondary root - branches on the primary root (lateral roots)

            4.   Seminal roots

                  a.   The primordia of the first two branch roots that form on the primary root

                  b.   Embryonic secondary roots

            4.   Adventitious roots - all roots arising from plant organs which are not roots

     D.   Morphology of growing root tips

            1.   Root cap

                  a.   Protects apical meristem

                  b.   Lubricates passage of root through soil

                  c.   New root cap cells are generated by the meristematic zone

                  d.   Seems to perceive gravity and controls the direction that the root grows

            2.   Meristematic Zone

                  a.   Root apical meristem occurs here

                  b.   Produces new root cap cells and new root cells in both directions

                  c.   Quiescent center

                        (1)  Slow rate of division of cells

                        (2)  Acts as a regulatory center for formatting the root's shape

                        (3)  Releases dividing cells (initials) to maintain root's shape

            3.   Elongation Zone

                  a.   Root begins to differentiate in this zone

                  b.   Elongation of the root cells mostly occurs here

            4.   Root-hair Zone (Zone of Differentiation)

                  a.   Final differentiation occurs here (all lengthening has been completed by this time)

                  b.   Absorption occurs only in this region of the root

                  c.   Final development of the primary tissues occurs here

                  d.   100 million new root hairs/day are produced in this region of a growing rye plant

II. Anatomy of Roots in the Completely Differentiated Root Hair Region

    A.   Differences between root and shoot apicies

           1.   Root Apex

                 a.   Root cap - covering over root tip

                 b.   Protoderm differentiates into the epidermis

                 c.   Ground meristem differentiates into the cortex

                 d.   Procambium is a cylinder

                 e.   Quiescent center - center of reduced cell division within the meristem zone

                  f.   Apical meristem produces the root cap, protoderm, ground meristem and                                          procambium

           2.   Shoot Apex

                 a.   Bud scales cover tip of stem

                 b.   Protoderm differentiates into the epidermis

                 c.   Ground meristem differentiates into the cortex

                 d.   Procambium is in individual strands arranged in a circle

                 e.   Not present

                 f.   Apical meristem produces the protoderm, ground meristem and procambium

    B.   Primary Meristem Tissues - still the same as in the stem

           1.   Protoderm - differentiates into the epidermis devoid of cuticle and guard cells

           2.   Ground meristem - differentiates into the cortex and endodermis

           3.   Procambium - is a cylinder and contains the pericycle

    C.   Epidermis

           1.   Single layer

           2.   Produces root hairs

                 a.   Lateral outgrowth of the epidermal cell wall

                 b.   Protoplasmic extension of the epidermal cell

                 c.   Exist and function for a few days to a few weeks

                 d.   DO NOT BECOME ROOTS

                 e.   Some plants have no root hairs ever

    D.   Cortex

           1.   Mostly storage parenchyma

           2.   Sometimes produces secretory cells/resin ducts

           3.   Endodermis

                 a.   Innermost layer of the cortex

                 b.   Casparian strip

                        (1)  Suberized

                        (2)  Complete ring of suberin on the radial and transverse cell walls

                        (3)  Forces water to pass through the endodermal protoplast

                        (4)  Plasmalemma fused to the casparian strip

    E.   Vascular cylinder

           1.   Pericycle

                 a.   Differentiated very early from the procambium

                 b.   Outermost layer of the vascular cylinder

                 c.   Remains meristematic

                 d.   Functions in:

                       (1)  Lateral root formation

                       (2)  Formation of sclerenchyma in primary roots

                       (3)  Formation of vascular cambium for secondary growth

           2.   Vascular tissue

                  a.   Differentiates from the internal portion of the procambium

                  b.   NO pith formed in dicots

                  c.   Pith CAN form in monocots

                  d.   Xylem tissue

                        (1)  Produced in radiating arms in the very center of the root

                        (2)  Called protoxylem poles

                        (3)  Xylem differentiates from the outside toward the center of the stem

                        (4)  Each of the arms is a protoxylem pole because that is where the first xylem
                              (protoxylem) begins to differentiate

                        (5)  Each of the protoxylem poles begin differentiating simultaneously

                  e.   Phloem tissue - small clusters of cells in between the radiating arms of the xylem

                  f.    Residual procambial cells remain undifferentiated between the xylem and phloem

                  g.   Those species with pith,  the pith is derived from the procambium

           3.   Dicots

                 a.   Five or less protoxylem poles

                 b.   Protoxylem begins to differentiate from the outside tips of the radiating arms

                       (1)   Diarch - 2 protoxylem poles

                       (2)   Triarch - 3 protoxylem poles

                 c.   Metaxylem develops after the protoxylem and is in the center of the root

           4.   Monocots

                 a.   More than five protoxylem poles

                 b.   Called polyarch

                 c.   Contains both protoxylem and metaxylem

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