BIOL0604 BIOL1309 EVOLUTIONARY DIVERSITY
5. Evolution of vascular plants
EMBRYOPHYTES or TERRESTRIAL PLANTS
TERRESTRIAL PLANTS
VASCULAR PLANTS
VASCULAR PLANTS
EVOLUTION OF VASCULAR PLANTS Key evolutionary changes: • Evolution of vascular system: XYLEM and
PHLOEM • Evolution of spores with thick protective wall
to reduce water loss • Progressive evolutionary reduction of
gametophyte; gametophytes become increasingly protected by and nutritionally dependent on sporophyte
PRIMITIVE CONDITION
ADVANCED CONDITION
EVOLUTION OF VASCULAR PLANTS
LACK OF LEAVES, ROOTS, etc.
PRESENCE OF LEAVES, ROOTS, etc.
COMPLEXITY OF THE PLANT BODY
PRIMARY & SECONDARY GROWTH
Transverse sections through the stem at different heights reveal developmental changes
PRIMARY & SECONDARY GROWTH
Primary growth: elongation of stem
Secondary growth: broadening of stem
PRIMARY GROWTH
Three main regions apparent at apex:
PROCAMBIUM
GROUND MERISTEM
PROTODERM
Further back from apex, cell differentiation occurs: Procambium vascular system
Protoderm epidermis
Ground meristem cortex & pith
PRIMARY GROWTH
SECONDARY GROWTH
Development of a ring of VASCULAR CAMBIUM (cells that are actively undergoing cell division)
Vascular cambium produces SECONDARY XYLEM on the inside, and SECONDARY PHLOEM on the outside
SECONDARY GROWTH
Epidermis breaks down, and is replaced by CORK tissue (or PERIDERM)
SECONDARY GROWTH
SECONDARY XYLEM = WOOD
SECONDARY PHLOEM + CORK = BARK
SECONDARY GROWTH
SECONDARY GROWTH
EVOLUTION OF SECONDARY GROWTH
PRIMARY GROWTH ONLY
PRIMARY GROWTH AND OFTEN SECONDARY
GROWTH
TRACHEARY ELEMENTS OF THE XYLEM • XYLEM: Complex tissue involved in water
transport and other functions • Several different cell types, including
TRACHEARY ELEMENTS for water transport
• Two basic types of tracheary element:
TRACHEIDS
VESSEL MEMBERS
TRACHEARY ELEMENTS OF THE XYLEM
• Evolution of vessel members from tracheids:
TRACHEARY ELEMENTS OF THE XYLEM
EVOLUTION OF TRACHEARY ELEMENTS
TRACHEIDS VESSEL MEMBERS
STELE ARRANGEMENT
• STELE: the arrangement of the xylem and phloem in the primary vascular system
• Basically two types: – PROTOSTELE: solid core of vascular
tissue; – SIPHONOSTELE: cylinder of vascular
tissue, surrounding non-vascular core
STELE ARRANGEMENT: Protosteles • Three types of protostele:
XYLEM
PHLOEM
STELE ARRANGEMENT: Siphonosteles Cylinder of vascular tissue surrounding non-
vascular core:
VASCULAR TISSUE
NON-VASCULAR TISSUE
Two basic types of siphonostele:
XYLEM
PHLOEM
PHLOEM
AMPHIPHLOIC SIPHONOSTELES
Phloem on the outside and inside of xylem
STELE ARRANGEMENT: Siphonosteles
Two basic types of siphonostele: AMPHIPHLOIC SIPHONOSTELES
STELE ARRANGEMENT: Siphonosteles
Two basic types of siphonostele:
XYLEM
PHLOEM
ECTOPHLOIC SIPHONOSTELES
Phloem only on the outside of xylem
STELE ARRANGEMENT: Siphonosteles
STELE ARRANGEMENT: Siphonosteles Two basic types of siphonostele: ECTOPHLOIC SIPHONOSTELES
EVOLUTION OF STELES
PROTOSTELE
AMPHIPHLOIC SIPHONOSTELE
ECTOPHLOIC SIPHONOSTELE
LEAF TYPES
Two basic types of leaf:
• MICROPHYLL • MEGAPHYLL
LEAF TYPES: Microphylls vs megaphylls
MICROPHYLL: Typically small
MEGAPHYLL: Typically large, often with expanded lamina
MICROPHYLL: Single vascular trace
MEGAPHYLL: Generally with complex arrangement of branching veins
LEAF TYPES: Microphylls vs megaphylls
MICROPHYLL: Associated with protosteles in stem MEGAPHYLL: Associated with more complex siphonosteles
LEAF TYPES: Microphylls vs megaphylls
MICROPHYLL: Strand of vascular tissue leading to leaf (LEAF TRACE) does not interrupt the stele MEGAPHYLL: Leaf trace creates LEAF GAP in stele
LEAF TYPES: Microphylls vs megaphylls
LEAF TYPES: Microphylls vs megaphylls
EVOLUTION OF MICROPHYLLS
EVOLUTION OF MEGAPHYLLS
EVOLUTION OF LEAF TYPES
LACK OF LEAVES
MICROPHYLLS
MEGAPHYLLS
ALTERNATION OF GENERATIONS
haploid (n)
diploid (2n) DOM
INAN
T
SPORIC MEIOSIS
EVOLUTION OF LIFE CYCLES
RELATIVELY LARGE GAMETOPHYTES
SMALL GAMETOPHYTES
GAMETOPHYTE
SPOROPHYTE
sperm
egg
zygote
spores
HOMOSPORY
LIFE CYCLES (1): Homospory
MEGAGAMETOPHYTE
MICROGAMETOPHYTE
SPOROPHYTE
sperm
egg
zygote
megaspores microspores
HETEROSPORY
LIFE CYCLES (2): Heterospory
EVOLUTION OF LIFE CYCLES
HOMOSPORY HETEROSPORY
• Evolutionary advantage of heterospory: spores develop into unisexual gametophytes which are not capable of self-fertilization
LIFE CYCLES
• MICROSPORES: ‘male’ spores • MEGASPORES: ‘female’ spores
• MICROSPORES borne in MICROSPORANGIA • MEGASPORES borne in MEGASPORANGIA
• MICROSPORES germinate to form
MICROGAMETOPHYTE • MEGASPORES germinate to form
MEGAGAMETOPHYTE
MICRO- and MEGA- therefore have different meanings:
• “small” and “large”, as in microphyll and
megaphyll; • “male” and “female”, as in microspore and
megaspore
LIFE CYCLES
MEGAGAMETOPHYTE
MICROGAMETOPHYTE
SPOROPHYTE
sperm
egg
zygote
megaspores microspores
HETEROSPORY
ENDOSPORY
EVOLUTION OF LIFE CYCLES
GAMETOPHYTE INDEPENDENT OF
SPOROPHYTE GAMETOPHYTE ENDOSPORIC
In heterosporous plants, the gametophytes are ENDOSPORIC: retained inside the spores