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Plant cells and tissues
Chapter 23
Apical meristems
 Meristem at the tip of the root or shoot
 Meristematic tissues are site of active cell division
 Indeterminate growth
Growth
 Primary growth is the extension of plants and addition of primary tissue
(epiderm, ground tissue, and vascular tissue)
 Secondary growth involves thickening of stem and root
Tissue systems
 Ground tissue system (simple tissues - one cell type)
 Vascular tissue system (complex tissue)
 Dermal tissue system (complex tissue)
Ground tissues
 Parenchyma
 Collenchyma
 Sclerenchyma
Parenchyma
 Most common cell in plants
 Wound healing and initiation of adventitious structures
 Photosynthesis
 Storage
 Secretion
Collenchyma
 Often cylindrical, elongated cells occurring in strands
 Unevenly thickened primary walls without lignin
Sclerenchyma
 Often lack protoplasts at maturity
 Thick, often lignified, secondary cell walls
 Fibers occur in strands or bundles
 Sclerids are variable and often branched
Vascular tissues
 Xylem
– Principle water conducting tissue
 Phloem
– Principle food conducting tissue
Xylem
 Derived from procambium in primary growth
 Derived from vascular cambium in secondary growth
Tracheary elements
 Elongated cells
 Have secondary cell walls
 Lack protoplasts at maturity
 May have pits in their walls
Vessel elements
 Have perforation plates occurring on end walls
 Vessel elements are joined end to end to form long tubes called vessels
Tracheids
 Less specialized
 No perforation plates
 More sensitive to air bubbles and subsequent cavitation
 Only water conducting cell found in seedless plants and gymnosperms
Sieve elements
 Cluster of pores called sieve areas connect protoplasts of adjacent sieve
elements
– Sieve cells are in gymosperms
 Sieve areas are in overlapping area of tapering cells
– Sieve tube elements are in angiosperms
 Sieve areas are on sieve plates located on the end walls
 Occur end on end as sieve tubes
Sieve elements
 Unlike tracheary elements, sieve elements have living protoplasts at maturity
– No ribosomes, Golgi apparatus, cytoskeleton
 Selective breakdown of cell rather than programmed cell death as in tracheary
elements
 Callose is polysaccharide that plugs sieve areas during injury and senescence
Sieve-tube elements
 P-protein (once called “slime”)
– P-protein is distributed along the walls of sieve-tube elements
– May seal phloem (“slime plugs”) during injury to reduce loss of phloem
contents
Companion cells
 Closely associated with sieve tube elements
 Delivery of informational molecules, ATP, and proteins to sieve tube element
 “Life support” for sieve tube element
Sieve cells
 Specialized cells called albuminous cells or Strasburger cells associated with
sieve cells
 Play role similar to companion cells
Dermal tissues
 Epidermis
– Dermal tissue of leaves, floral parts, fruits, seeds and roots/stems before
secondary growth
 Peridermis
– Dermal tissue of roots that have undergone secondary growth
Epidermis
 Compactly arranged, providing mechanical protection
 Aerial epidermal cells covered with cuticle
 Typically lack chloroplasts
 Bulk of cells are unspecialized
 Specialized cells called guard cells and trichomes
Guard cells
 Regulate openings or stomata in leaves for gas exchange
 Contain chloroplasts
 Guard cells often associated with subsidary cells
Trichomes
 “Hairs” on leaves
 Increase reflection of solar radiation
 Lowers leaf temperatures
 Lowers rate of water loss
 Absorb water and nutrients in epiphytes
 Secrete salt in halophytes
 Provide defense against insects
Periderm
 Replaces epidermis in stems and roots that undergo secondary growth
 Arranged compactly except for at the area of the lenticels where gas exchange
can take place
– Consists of cork (non-living and suberized), cork cambium and phelloderm
(living parenchyma tissue)