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