Lecture #15 - Gymnosperms
Fall 2001
Gymnosperm refers to the mode of seed production .... in this case naked-seeded plants.
** As a group, gymnosperms included several unrelated evolutionary lines with divergent forms
many of the extant species and genera have a markedly disjunct geographic distribution ... and
many species with a limited distribution.
*** - The geological record of gymnosperms extends from the Devonian (between 350 and 400
mya) during the Paleozoic Era.
*** - Each of the four extant conifer families has a fossil record extending back from the
Mesozoic Era
and the fossil evidence shows a much wider geographic distribution than that of
the living conifers.
*** - Leaves identical to Ginkgo biloba occurred as far back as the Permian
some 230-280 mya
(lower Paleozoic).
*** - Gymnosperms consist of only 500 or so species, about 61 genera and 9-11 families. They
are quite different from angiosperms in a number of ways:
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Gymnosperms are naked seeded and lack double fertilization
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They lack xylem vessels (possess tracheids only) except in gnetophytes
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They lack companion cells in the phloem (possess sieve cells only)
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They possess a many-celled or many-nucleated female gametophyte
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Archegonia (multicelled compartments in which an egg is produced) are present in the
female gametophytes
** - Although the numbers of species and genera are few, gymnosperms are important from an
economic and biological standpoint
and include such groups as cedars, firs, hemlocks,
redwoods, cypress, spruces, junipers, pines etc...
** - Classification of Gymnosperms:
Paleobotanical evidence suggests several long-distinct evolutionary lines in the evolution of the
gymnosperms and that the seed-producing habit arose several times during the history of land
plants (i.e., polyphyletic).
** - Consequently, as in the angiosperms, there are several classification schemes for the
gymnosperms, and each interpretation depends on individual weighting of data .....
Most Common Scheme -
Division Pinophyta (Gymnosperms)
Three Major Subdivisions (not to be confused with subclasses)
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Cycadicae - Seed Ferns & Cycads
(http://scitec.uwichill.edu.bb/bcs/bl14apl/gym2.htm) - for cycads
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Pinicae - Ginkgos & Conifers
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Gneticae - Gnetophytes
I. Subdivision Cycadicae: Palm-like to fernlike plants; leaves mostly compound; wood soft and
loose without vessels; large (some 18 meters tall; male cones = microstrobili which produce
pollen and megastrobili - which produce eggs ... on separate plants.
** The cycads today are a small family of plants scattered throughout the tropics and of no
particular economic importance. They reached their prime of complexity and diversity something
like 150 million years ago and since then they have gone into decline. Now only a few species
remain.
http://scitec.uwichill.edu.bb/bcs/bl14apl/lab4.htm
** There are three classes in this subdivision ---- we will mention two here:
(a) Seed Ferns - may have given rise to the first angiosperms; treelike with fernlike fronds;
pollen producing organs and seeds borne on the leaves; all are extinct.
(b) Cycads - about 10 genera and 100 species; appeared 285 million years ago in the
Carboniferous Period numerous during the Mesozoic; leaves pinnately compound; forming a
crown at the tip of the stem; diecious; seeds born on megasporophylls aggregated into strobili;
motile sperm.
The fossil history of the Cycads extends back over 200 million years. The living species are the
most primitive extant seed plants. They possess unbranched trunks and some grow to more than
50 ft. tall. Their seeds are brightly colored, usually red.
http://web1.manhattan.edu/fcardill/plants/gymno/pinophy.html
II. Subdivision Pinicae (Conifers & Ginkgos) - plants without vessels & relatively dense includes two classes:
** - Ginkgoateae - This division has one living member which probably originated about 250
million years ago. Ginkgo is similar to cycads in reproductive structures and gametophyte
development but the leaves are distinctly different. It is a hardy tree which can live up to 1000
years. After the first 10 to 20 years of vertical growth, laterally spreading branches develop.
Leave are deciduous; microstrobili catkinlike; sperm are motile. One species left from a genus
that has changed very little in 80 millon years (Ginko biloba). May reach 30 m in height.
Class Pinate - (conifers) - mostly trees ... some shrubs; date back about 290 million years to the
Carboniferous Period; usually evergreens; leaves typically scalelike or needle-like; seed cones
contain flattened ovules attached to the upper surface of an ovuliferous scale subtended by a
bract; freely branching stems.
** Two important orders in this Class = Coniferales and Taxales
** - Coniferales includes pines, spruce, hemlock, larch, fir, cypress, sequoias and cedars
http://web1.manhattan.edu/fcardill/plants/gymno/conifers.html (to find these orders & families)
** - Taxales = Yew Family - Family Taxaceae
Subdivision Gnetophyta (gnetophytes) - problematic gymnospermous plants of interesting
morphology ... and unusual structure; embryos with 2 cotyledons as opposed to one in other
groups; compound male and female cones (whereas male cone in other groups is simple).
Most prominent group are in the group called Getales = woody vines or occasionally shrubs or
trees. Leaves opposite; pinnately netted
resembles many dicots; vessels present in wood!
Strobili not cone-like; 1 family and 3 benera; inhibats tropical rainforests
or in this species -
found in desert habitats.
Gnetum right appears to be related to angiosperms but have their own unique
specializations. See:
http://www.science.siu.edu/landplants/Gnetophyta/gnetophyta.html