植物學教案-根結構

1、The Primary StructureEpidermisCortexVascular cylinderEpidermisFormed by the protodermClosely packed parenchyma cells with a large numbers of root hairs (tubular extensions of the epidermal cells)Absorbing surface.CortexFormed by the ground meristem.Composed of parenchyma cells which occupy the large

2、st cross-sectional area of a primary root. Consists of three concentric layers:HypodermisStorage Parenchyma CellsEndodermis Cortex Hypodermis The outermost layer of packed parenchyma cells which resemble epidermis. Sometimes with suberized cell walls (beyond the root hair zone).Retain water and nutr

3、ientsCortex Storage Parenchyma CellsParenchyma cells with numerous intercellular spaces and plasmodesmata. Storage of starchRadial conduction by way of the protoplasts and cell walls. Cortex EndodermisThe innermost layer, compactly arranged, contain Casparian strips on anticlinal walls. Radial condu

4、ction by way of the protoplasts.Vascular CylinderPericyclePrimary XylemPrimary PhloemPithVascular Cylinder Pericycle A meristematic layer of parenchyma cells one to several cells thick.Contributes to the cork cambium and vascular cambiumProduces lateral roots (endogenous).Vascular Cylinder Primary X

5、ylemForms radiating ridges (Diarch, Triarch, Tetrarch, Polyarch)Two parts:protoxylem: differentiates early, while adjacent cells are still elongating. Metaxylem: differentiates late, after adjacent cells have completed their elongation.Exarch: primary xylem differentiates from outside to inside.Vasc

6、ular Cylinder Primary PhloemNestled strands between the ridges of xylem Two parts:ProtophloemMetaphloemExarchVascular Cylinder PithParenchyma tissue in the center, located interior to the vascular bundlesIn root, pith mainly occurs in monocots and a few dicots.The Secondary StructureOriginates from

7、secondary meristem (phellogen and vascular cambium)Occurs in gymnosperms and most dicots.Two parts:Periderm: from phellogen (cork cambium)Secondary Vascular Tissues: from vascular cambiumFormation of PeridermPericycle cells divide periclinally into two layers.Phellogen originates from the outer sist

8、er cells of pericycle. The inner sister cells give rise to the secondary cortex. The epidermis and cortex die and are sloughed off.Formation of Secondary Vascular TissuesVascular cambium originates from part of pericycle cells and procambial cells that remain between primary xylem and primary phloem

9、Secondary xylem and phloem are added to the root and vascular rays are producedFinally, the primary phloem is crushed but fibers may remain.Overall View of Secondary StructureRemnants of the epidermis and cortex or nonePeridermPericycle (secondary cortex)Primary phloem (fibers and crushed cells)Secondary phlo