Ferns
Gymnosperms
Angiosperms
Gnetum
• Description: (some apply to other Gnetales)
– Monoecious or dioecious
– Stems woody (mainly vines), swollen nodes, vessels (up to 300
um) with vestured pits and fibers, high photosynthetic and
transpiration capacities, syringaldehyde lignin, tunica presence
outer apical meristem (1 cell wide not two)
• Vessels derived from tracheids with circular pits (Angiosperms derived
from tracheids with scalariform pits)
– Leaves evergreen, opposite with reticulate venation and petioles,
often drip tips, up to 10 cm wide
• Fine venation is derived from fibers not vascular strands (Angiosperms)
Gnetum
• Description: (some apply to other Gnetales)
– “Flowers” in strobili: Reduced megagametophytes, Double
fertilization (but no endosperm), Possibly entomophily, Outer
envelope encloses ovule
– Seeds drupe like in brightly colored fleshy “fruit”
– Cotyledons 2 and epigeal
– Ectomycorrhizal
Gnetum
• But…sometimes gymnosperm like
– Xylem can have tracheids with torus/margo pit
– Root morphology
– Circular border pitting
Habitat
• Gnetum: Tropical lowland rainforest
• Ephedra: arid
• Welwitschia: desert
Distribution
• 40 species: 10 S. America, 1 W. Africa, 20-25
Asia
Historical biogeography
• Won and Renner (2006):
– Divergence began in late Oligocene to midMiocene (~25 mya)
– Asian clades diverging in Miocene to Pliocene (422 mya)
– When did Gondwana break up? (~200-250 mya)
• African spp not a Gondwana relict!
• Water dispersal is likely
Gnetum
• First evidence of horizontal gene flow (Won
and Renner 2003)
– Gnetum and the asterid Petunia (2-5 mya): nad1
intron 2
Evolution of efficient xylem
Evolution of efficient xylem
Questions
• Based on habit, xylem, and leaves, is Gnetum
a weedy-disturbance specialist with fast
photosynthetic and water transport rates?
– Conducted field measures of hydraulic
conductivity, photosynthesis and stomatal
conductance in PNG with 2 tree and 2 liana
Gnetum species (as well as a Podocarpaceae and
11 Angiosperms)
Questions
• Based on habit, xylem, and leaves, is Gnetum
a weedy-disturbance specialist with fast
photosynthetic and water transport rates?
– Answer: No, actually performed similarly to
Podocarpaceae, except for Gs (transpiration
rates), Ks (sapwood specific conductivity), and HV
(Huber value; sapwood area/leaf area)
– Vessels and pinnate-veined leaves may allow for
greater light capture and growth under low light