Table Of ContentSeed Plants – The Gymnosperms
The seed plants evolved from fern-like non-seed ancestors. Several changes
occurred to make this novelty possible. First, two types of spores, large megaspores and
small microspores, appeared. This change is illustrated in Selaginella and some aquatic
ferns. In both, the gametophytes are reduced in size, developing within the spore walls. The
male gametophyte developing within the microspore wall became the pollen. The female
gametophyte developed within the spore wall, and the spore was retained within the
megasporangium. For fertilization to occur pollen was carried by wind to the
megasporangium, the grains germinated as a tube and the male gametes moved to the egg
cell. After fertilization, the embryo developed inside of the megasporangium, now called
the ovule. The fertilized ovule then became a seed, with an embryo inside.
This new life cycle had several advantages. First, the protected pollen grain
was blown by wind to the site of germination, reducing the requirement for water and
permitting these plants to sexually reproduce in much drier conditions. Secondly, the
development of the seed provided a means of protecting the embryo against dessication and
the storing of the embryo in dormancy until ideal conditions would trigger germination.
Finally, modifications of the seed promoted dispersal by wind or animals.
Plants with this life cycle are called gymnosperms because the ovule/seed is
produced on a leaf-like structure and is unprotected, or naked. Gymno- = naked and -sperm
= seed. We will see details of this modification and this new life cycle in two plants native
to south Florida. There are four groups of living gymnosperms: the conifers, the cycads,
Gingko, and Gnetum and its relatives. Ginkgo is represented by a single species: Gingko
biloba. It does not grow in south Florida, but it is sold in health food stores as a tonic to
improve cerebral circulation and memory in aging.
The Conifers
These trees are enormously important, as the source of softwood timber used in
wood-based construction and the source of fiber in producing most of the world’s paper. They
are dominant in certain forests at temperate and higher latitudes, as in northern latitudes and the
northwest of the United States. Most of the coastal areas of south Florida, on the limestone
ridge, were covered with a pine forest, called the pine rocklands. Little of this forest remains,
having been replaced by agriculture and then commercial development. Conifers vary in their
leaves and particularly in their female cones; some being reduced to look more superficially
like a berry. They all share the same basic life cycle, that of a pine tree given as an example
below. Three conifers native to south Florida are described in detail, and several exotic species
are mentioned briefly.
Dade County Pine—
Pinus elliottii var. densa. This
extreme southern variety of the slash
pine grows in a few remaining stands
on the rock ridge of south Florida. Its
wood is very resinous, extremely
hard when dry, and very resistant of
termite attack. It was used in the
construction of homes and boats well
into the last century. Dade County
pine grows in the Ecosystem Preserve
and the parking lot just to the east. A
couple of trees also grow in the small
conifer collection NE of the north
parking lot.
Dade County Pine, continued
The fragile male cones
and small purplish female cones
develop in January-February. In a
forest the air is yellow with the wind-
carried pollen. After fertilization the
female cone scales swell and close.
Then the cones develop for the entire
year, and open to release the winged
seeds prior to the rainy season (May)
the next year. The trees always have
female cones in some stage of
development, but the male cones soon
fall off the tree after they shed their
pollen. The Dade County Pine is a
member of the pine family (Pinaceae)
along with all pines and the firs, such as
the Frazer’s fir from the Appalachian
Mountains on sale before Christmas.
Phylum Gingophyta: The Ginkiphyta Procedure – examine pine twigs and
consist of one species, Ginkgo biloba leaves
(Maindenhari plant), a large dioecious tree that
1. Examine pine twigs having leaves
does not bear cones. Ginkgo are hardy plants
(needles) and a terminal bud. Notice
in urban environments and tolerate insects,
the number of needles; the length and
fungi, and pollutants. Males are usually
number of leaves distinguishes many
planted because females produce fleshy,
of the species Pinus.
smelly, and messy fruit that resembles cherries.
Ginkgo has not been found in the wild and would
Questions
probably be extinct but for its cultivation in
1. How are the needles arranged?
ancient Chinese and Japanese gardens.
2. How many leaves are in a bundle?
3. How are pine leaves different from
Phylum Gnetophyta: This gnetophytes (71
those of deciduous plants?
species in 3 genera) include some of the most
4. Why are pines called evergreens?
distinctive (if not bizarre) of all seed plants. They
5. How do the structural features of pine
have many similarities with angiosperms, such as
leaves adapt the tree for life in cold,
flowerlike compound strobili, vessels in the
dry environment?
secondary xylem, loss of archegonia, and double
fertilization.
Pine life cycle
In seed plants, the gametophyte gneration is greatly reduced. A germinating pollen grain is
the mature microgametophyte (male cones) of a pine. Pine microsporangia are borne in
pairs on the scales of the delicate pollen-bearing cones. Megagametophytes (female cones),
in contrast, develop within the ovule. The familiar seed-bearing cones of pines are much
heavier than the pollen-bearing cones. Tow ovules, and ultimately two seeds, are borne on
the upper surface of each scale of a cone. In the spring, when the seed-bearing cones are
small and young, their scales are slightly separated. Drops of sticky fluid, to which the
airborne pollen grains adhere, form between these scales. These pollen grains geminate,
and slender polled tubes grow towards the egg. When a pollen tube grows to the vicinity of
the megagametophyte, sperm are released, fertilizing the egg and producing a zygote there.
The development of the zygote into an embryo occurs within t the ovule, which mature into
a seed. Eventually, the seed falls from the cone and germinates, the embryo resuming
growth and becoming a new pine tree.
Pine Life Cycle diagram
Procedures and questions about
conifer reproduction
Procedure – examine pine cones Procedure – examine a pine seed
1. Examine young living or 1. Examine a prepared slide of a
preserved ovulate cones. These pine seed. Locate the embryo,
cones will develop and enlarge seed coat, and food supply.
considerable before they are Seeds are released when the
mature. cone dries and the scales
2. Examine a prepared slide of a separate. This usually occurs
young ovulate cone ready for 13-15 months after pollination.
pollination. Each ovuliferous 2. Examine some mature pine
scale of the female cone bears seeds, noting the winglike
two megasproangia, each of extensions of the seed coat.
which produces a diploid
megaspore mother cell. Each Questions
megaspore mother cell undergoes 1. On which surface of the scale
meiosis to produce a megaspore are the seeds located?
that develops into a
megagametophyte. A 2. How large in a staminate cone
megagametophyte and its compared to a newly pollinated
surrounding tissues constitute an ovulate cone? A mature ovulate
ovule and contains at least one cone?
archegonium with an egg cell.
3. Examine a prepared slide of an 3. What is the make gametophyte?
ovulate cone that has been
sectioned through an ovule. An 4. What is the female
ovule develops into a seed. gametophyte?
4. Examine a mature ovulate cone
and notice its spirally arranged 5. What is the function of the
ovuliferous scales. These scales winglike extensions of a pine
are analogous to seed?
microsporophylls of staminate
cones, but ovuliferous scales are 6. How are other gymnosperms
modified branches rather than similar to pines?
modified leaves. At the base of
each scale you’ll find two naked 7. How are they different?
seeds. Notice that the seeds are
exposed to the environment and
supported (but not covered) by an
ovuliferous scale.
Bald Cypress—Taxodium distichum. This is a conifer in another family, the Taxodiaceae.
Its female cones are much smaller and the individual scales are rounded to produce a round
cone. It is a swamp tree, growing in stands throughout the southeast. It was once common
in a strip of swamp forest down the southeast coast of Florida, and more common along the
west coast, as in the Big Cypress National Preserve. Bald Cypress trees were planted on
pond margins at FIU soon after it opened. We now have some bald cypress “domelets”, with
cypress knees (the pneumatophores that assist in oxygen uptake to the roots) and Everglades
wading birds sitting on branches. The bald cypress is unusual among conifers in that it loses
its short needle foliage during the winter months. Few of the original cypress domes
remain; the majority of these swamp forests were logged before and during the Second
World War, partly for the construction of PT boats.
Phylum Cycadophyta: the cycads
These gymnosperms are no longer widely distributed, only found in mostly dry tropical
regions, but they were once dominant plants. These were the primary food of the large
herbivorous dinosaurs. Most cycads are extremely tough, thorny, and often very toxic.
Fairchild Tropical Garden, and the adjacent Montgomery Botanical Center, have the largest
cycad collection in the world. Cycads have life cycles similar to the conifers, but certain
details (as the flagellate male gametes) are different. Cycad plants are female (producing
long-lived female cones) or male (producing ephemeral male cones). We illustrate the
cycad life cycle with the example of the coontie, Zamia pumila, and describe a few cycads
commonly encountered in south Florida (and on campus).
Zamia Life Cycle
Coontie—Zamia pumila. The
coontie is the only cycad native to
the United States, growing in
south Florida Pinelands. Its
rhizomes are full of starch, which
was the source of the first
manufacturing industry in south
Florida. The "trunks" were
ground up to release the starch,
the starch was then washed to
remove the toxic cycasin, and the
product dried and ground. Florida
“arrowroot” was then shipped up
the east coast for cooking and
stiffening the collars of Victorian
shirts. The coontie is a small
plant, less than half a meter high.
It grows on campus in the
Ecosystem Preserve, the Campus
Security Compound, and by the
Conservatory. Recently, the
remarkable discovery was made
that the coontie is pollinated by
beetles, that feed on both the male
and female cones.
Seed Plants – the Angiosperms – Flowering Plants
The angiosperms are seed plants, similar to gymnosperms, but with some important
evolutionary modifications. Flowers are reproductive organs derived from leaf-like
appendages. The relationship of the accessory flower organs, petals and sepals, is obvious.
The stamens and pistils can also be seen in development to originate from leaf-like
structures. In the flowering plant life cycle, the male gametophyte which develops within
the microspore wall into a pollen grain are even more reduced than in the gymnosperms.
Its movement to the ovule is often aided by appearance and scent, attracting pollinators. The
female gametophyte develops as the embryo sac, within an ovule, and within a new
structure: the ovary. In pollination the pollen grain germinates on the stigma of the pistil
and grows down the length of the style to the opening of the ovule. After fertilization, the
embryo sac and ovule develop into the seed. A second fertilization produces a nutritive
tissue, the endosperm, that surrounds the embryo. At maturity, the ovules, or seeds, are
protected within the ripened ovary wall to become a fruit. The fruit, fleshy or dry, aids in
dispersal.
Peduncle – flower stalk
Sepals – the lowermost or outermost whorls
of structures, which are usually leaflike and
protect the developing flower; the sepals
collectively constitute the calyx.
Petals – whorls of structures located inside
and usually above the sepals; the petals
collectively constitute the corolla.
Androecium – the male portion of the plant;
consists of stamens, each of which consist of
a filament atop which is located an anther;
inside the anthers are pollen grains which
produce the male gametes
Gynoecium – the females portion of the
plant; consist of one or more carpels, each
made up of an ovary, style, and stigma; the
ovary contains ovules that contain the female
gametes. The term pistil is sometimes used
to refer to an individual carpel or a group of
fused carpels.
More information about Angiosperms
Two classes of angiosperms Flower symmetry
The sepals and petals are usually the most
conspicuous parts of a flower, and a variety
Monocots
of flower types are described by the
• One cotyledon per embryo
characteristics of the perianth (combined
• Flower parts in sets of three
calyx and corolla). In regular
• Parallel venation in leaves
(actinomorphic) flowers such as tulips, the
• Multiple rings of vascular bundles in
stem members of the different whorls of the flower
• Lack a true vascular cambium (lateral consist of similarly shaped parts that radiate
meristem) from the center of the flower and are
equidistant from each other. The flowers are
Dicots radially symmetrical. In other flowers such
• Two cotyledons per embryo as orchids, one or more part of at least one
• Flower parts in sets of 4 or 5 whorl are different from other parts of the
• Reticulate (i.e., netted) venation in same whorl. These flowers are generally
leaves bilaterally symmetrical and are said to be
• One ring or vascular bundles in stem irregular (zygomorphic).
• Have a true vascular cambium (lateral
meristem)
A radially symmetrical flower
Photo by Gita Ramsay
A bilaterally symmetrical (irregular) flower
Photo by Gita Ramsay
Description:It does not grow in south Florida, but it is sold in health food stores as a tonic to improve cerebral . Seed Plants – the Angiosperms – Flowering Plants.
