BIOLOGY 3404F
EVOLUTION OF PLANTS
Fall 2008
Lecture 14
Tuesday Nov 11, 2008
Chapter 18, Gymnosperms I:
Cycadophyta, Ginkgophyta & Gnetophyta
What is a gymnosperm?
• Plants that produce naked ovules, exposed on
the surface of sporophylls; no ovary wall
• There are about 720 species of gymnosperms,
versus 235,000 species of angiosperms.
• Ovule: a megasporangium (produces female
megaspore) within 1-2 layers of tissue called
the integument(s)
[Pteridosperm
-ophyta]
Late Devonian (370 MYA) Elkinsia ovules and sporophylls (cupules)
Ovules versus ovaries
• “Female portion” (gynoecium) of a flower may
contain one or more carpels (separate or fused),
each considered to be a modified leaf; these are
found in angiosperms
• “Pistils” (one or more carpels) consist of ovary,
style and stigma; pollen lands not on ovule as in
gymnosperms but on tip of stigma – “indirect
pollination”
Late Devonian Archaeosperma ovules (a-b); fossil “seed” (360 MYA)
[Pteridospermophyta]
Gradual enclosure of early ovules in Pteridospermophyta
Progymnosperms: Aneurophytales
and Archaeopteridales
• Progymnosperms appeared in the Devonian (380 MYA).
• These resemble trimerophytes but produced bifacial vascular
cambium (see Fig. 18-6 and paragraphs below it), which
produces secondary phloem and xylem - i.e., true wood.
• Examples are Aneurophyton and Archeopteris (= Callixylon, the
name for its trunks); the latter formed large trees in southern
Ontario!
• What's missing? Seeds!
Archaeopteris, and a view of its foliage and sporangia
[Wood of Archaeopteris is called Callixylon]
Callixylon newberryi wood (polished slab), Devonian, New Albany
Shale, Liter's & Clark County, Indiana; same, 50X
(photos by Rick Schrantz, Kentucky Paleontological Society:
http://www.uky.edu/OtherOrgs/KPS/pages/fossilphoto.html)
A radial section of Callixylon
newberryi wood
Wood of Callixylon huronensis from Kettle Point, Ontario (~370 MYA)
Wood of Callixylon, probably C. huronensis, Kettle Point, Ontario
Progymnosperms: Aneurophytales
Aneurophyton germanicum, from
middle Devonian in Rhineland,
Germany. Aneurophyton had a
protostele, secondary growth, was
highly branched and possibly
shrubby, and reproduced by spores
(homosporous).
Photo by Volker Wilde, from
http://www.senckenberg.unifrankfurt.de/fis/palbot.htm
Gymnosperms
• 5 phyla of “gymnosperms” are Pteridospermophyta,
Coniferophyta, Ginkgophyta, Cycadophyta, Gnetophyta [but see
recent evidence placing gnetids in Coniferophyta]
• Pteridospermophyta (e.g., Elkinsia, Archeosperma) appeared in
the Devonian (365 MYA).
• Cordaites is regarded as a primitive member of the
Coniferophyta; it was present in the Carboniferous (300+ MYA).
• Two of the remaining “gymnosperm” phyla, Cycadophyta and
Ginkgophyta, appeared in the Permian (290-245 MYA); the
Gnetophyta appear to have come in the Cretaceous (130 MYA).
Pteridospermophyta (seed ferns)
• Pteridospermophyta (e.g., Elkinsia,
Archeosperma) also appeared in the Devonian
(365 MYA)! The Devonian was a busy time.
• Medullosa (Carboniferous) looked like a tree
fern, but produced seeds in cupulate ovules.
Carboniferous seed fern Medullosa
CYCADOPHYTA:
Currently, these are a small group of living fossils, primarily
tropical/subtropical, (dominant during Mesozoic, or cycaddinosaur age, +/- 200 MY bp); 0.5-3 (18) m tall; 11 genera,
275 species [140 in text].
1. Cycads have fern-like or palm-like foliage: pinnate, often with
"circinate vernation" (unfurling). Common name of some is
"sago palms"
• [Sago flour is prepared from pith of Cycas circinalis in
India & Sri Lanka and C. revoluta in Japan; but also from
true palms (Metroxylon and some species of Arenga and
Caryota), which are monocotyledons of the Anthophyta].
Cycads II
2. Stout stem (mostly subterranean) is mostly pith - despite
vascular cambium, there is very little secondary phloem or
xylem; growth is slow.
3. Leaves are arranged spirally on stem (usually in a dense tuft
at the apex).
4. Many inhabit dry areas such as dry deserts of Australia;
Zamia lives in sandy spots in southern Florida among
palmettos. Primary roots are well developed (up to 12 m);
deep roots and fleshy stems store water.
Cycads III
5.Secondary, coralloid roots grow on or above
soil, like cypress "knees", and contain
symbiotic cyanobacteria (Anabaena) that fix
atmospheric nitrogen.
[Name 2 other symbioses between plants with
nitrogen-fixing cyanobacteria]
Cycads IV
6. Heterosporous (goes without saying in seed plants); dioecious
(2 homes), so one plant produces male strobili and another
produces female, or ovulate, cones. The microgametophyte is
wind-dispersed pollen; female cones produce several to many
ovules, spirally on an axis; these produce a "pollination drop"
(a drop of liquid at micropyle); pollen landing on the
pollination drop adheres, and then is sucked into the micropyle
to a pollen chamber as the drop dries and shrinks; then grows a
pollen tube, down which the sperm swim using their many
flagella (undulipodia) to reach and fertilize the egg.
Cycads V
7.Although there may have been several
archegonia per ovule, only one usually
develops into a zygote in the mature seed.
The fruit is a fleshy cone
Zamia integrifolia [= Z. floridana, named Z. pumilla in your text]
in Florida; large grey female cones and smaller brown male cones
Female Encephalartos ferox from Africa
Female Cycas siamensis with seeds on upper surfaces of sporophylls
Sperm of cycads swim to egg (left) after being delivered nearby in
a pollen tube (right)
GINKGOPHYTA:
A real living fossil (I've collected Mesozoic fossil Ginkgo
leaves in western Canada) with only 1 genus and
species left of its once larger lineage.
1. "Maidenhair tree" has been cultivated for centuries in
temple gardens of the orient (first China, then
imported to Japan) then imported to botanical gardens
of Europe; may no longer grow wild anywhere in the
world, although there are reports of a few "wild" trees
south of the Yangtze River in China. Makes a great
city tree (especially males) because it is highly
resistant to smog, and has few pathogens.
Ginkgos II
2. A tree to 30+m, with good secondary growth from cambium (or
"vascular cambium"), producing secondary xylem (= wood)
internally and 2° phloem externally; outer layer of cortex
differentiates to form a cork cambium or phellogen, producing a
periderm, or bark. Leaves and microsporangia/ovules
(dioecious) are produced on short shoots (spurs) from long
shoots (branches): gives the Ginkgo a characteristic shape (stem
+ horizontal, unbranched branches).
3. Leaves are fan-shaped, with palmate venation, and may have
central notch (Ginkgo biloba); stomata are produced on the
underside.
Ginkgos III
4.Pollen (microgametophytes) is produced in
microstrobili/microsporangia and is winddispersed to the ovules of a female tree. Ovules
are paired on a short stalk, or peduncle; each
produces a pollination droplet, sucks the pollen
in to the pollen chamber, where a pollen tube
grows; sperm are flagellated and swim to
fertilize egg.
Ginkgos IV
5. Although there are 2 (-4) archegonia per ovule,
only one zygote develops in the mature seed, and
only one of the two ovules develops into a mature
"fruit" like a cherry but pale greenish yellow - the
integument swells and is fleshy; the flesh causes
nausea, but is unlikely to be eaten because is
stinks, from butanoic and hexanoic acids; the
kernel may be roasted and eaten.
Development of male gametophyte and
gametes in ginkgo
http://www.xs4all.nl/~kwanten/history.htm
Ginkgo Petrified Forest State Park,
Vantage, Washington
Millions of logs were swept by a
volcanic mud-flow into “Lake
Vantage”, covered in sediments,
and petrified (agatized and opalized); 50+ genera of trees are
represented in this 16 MYA forest. Some are still found in
Western North America (WNA); others in ENA or Asia.
GNETOPHYTA:
70 species in 3 genera that are very poorly known
- primarily desert-dwellers. The genera are
different enough from each other that each is
classified in its own order, and may be even less
related than that. Originate in the Cretaceous
(~130 MYA). [or before!]
1.Xylem of Gnetophytes contains vessel members
(together form vessels).
Gnetophytes II
2. Ephedra are shrubby or trailing pants with green, jointed stems
and minute, ephemeral scale-like leaves; looks like Equisetum
(but produces seeds not spores, and has completely different
stem cross-section, with secondary growth).
3. Occur in SW North America, South America, southern Eurasia
and north Africa. 35-40 species.
4. May be monoecious or dioecious;
microstrobili/microsporangia produce wind-borne pollen and
megastrobili have 4-7 pairs of bracts, with the upper 1-2 with
an ovule in the axil - usually only one develops.
Gnetophytes III
5. The ovule may contain 2-3 archegonia, and the
micropyle actually extends as a neck, and exudes a
pollination droplet, sucks in several pollen grains to
the pollen chamber. Pollen germinates to form a
pollen tube that grows down the neck of the
archegonium, and one sperm nucleus emerges to
fertilize the egg - it is non-motile as in conifers and
angiosperms.
6. Seed is hard and black, surrounded by red fleshy
bracts of megastrobilus.
Gnetophytes IV
7. Ephedra is the source of ephedrine, a drug like epinephrine
(adrenaline) used to treat asthma, emphysema and hay fever; and
used traditionally (e.g., by SW Indians) to treat venereal diseases
(E. antisyphilitica).
8. Gnetum is a genus of about 30 species of vine to tree-like plants
growing in Brazil, tropical west Africa, wet parts of India, and
south-east Asia. Gnetum has large, leathery leaves like a dicot,
and fleshy fruits.
9. Welwitschia mirabilis is the only representative of its group, and
grows in the ultra-deserts of SW Africa. See Raven et al. and lab
notes.
Gnetum leaves and megasporangia
Gnetum, leaves and microsporangia
Gnetum seeds - these are fleshy, but are not true fruits
Ephedra, male plants and “microsporangiate strobili”
Ephedra, female plant with seeds
Welwitschia mirabilis, in Namibian desert, large plant with seeds
Welwitschia, male (microsporangiate) strobili (left) and female
(megasporangiate/ovulate) strobili, with hemipteran (right)