Gymnosperms
Gymnosperms
Evolutionary development of the seed
Seeds consist of an embryo
already packaged within the seed, and nutritive tissue surrounded by a
protective coat.
1.
A seed contains a
well-developed multicellular young plant with
embryonic root, stem, and leaves already formed, whereas a plant spore is a
single cell.
2.
The parent plant
protects and supports the young plant in the seed during its development;
spores do not receive such attention.
3.
A seed contains a
food supply. After germination, in which the young plant begins to grow and
establish itself as an independent plant, it is nourished by food stored in the
seed until it becomes self-sufficient. Because a spore is a single cell,
minimal food reserves exist to sustain the plant that develops from a
germinating spore.
4.
The seed is
protected by a multicellular coat. Seeds (and spores)
can survive for extended periods of time at reduced rates of metabolism and
then germinate when conditions become favorable.
5.
Seeds and seed
plants have been intimately connected with the development of human
civilization. From prehistoric times, early humans collected and used seeds for
food. The food supply stored in seeds contains proteins, oils, carbohydrates,
and vitamins that are nourishing for humans as well as for germinating plants.
6.
Because seeds are
easy to store, provided they are kept dry, humans could collect seeds during
times of plenty, and save them for times of need. Few other foods can be stored
as conveniently or for as long as seeds.
7.
The elimination
of the necessity of water to transport the sperm grom
the microgametophyte to the megagametophyte
for fertilization to occur.
8.
This development
made it possible for seed plants to complete their life cycles in relatively
dry environments, compared to those of non-seed-producing vascular plants.
Seed production is an
adaptation of great significance for the survival and dispersal of plants. In
fact, this was part of the competititve advantage
that allowed the gymnosperms to supercede the other vascular plants as the
dominant type of vegetation on land. Only the later evolution of flower and
fruit allowed another group of seed plants (the angiosperms) to displace the
gymnosperms from their preeminent position.
Gymnosperms
are seed-bearing plants that lack the combination of specialized features that
characterize the flowering plants. The name gymnosperm,
means naked seed. (e.g. gymnasium); that is, the seeds
are not enclosed within fruits.
Gymnosperms, then, are all
fruitless seed plants. And they are made up of a heterogeneous group of plants
characterized by the production of naked seeds. Estimates form fossil records
indicate that gymnosperms must have evolved approximately 300 million years ago
from non-seed producing
ancestors of the extinct division of Progymnospermophyta,
which were fern-like in appearance.
1. They lack the folded, marginally-sealed
carpels that characterize the flowering plants.
2. The pollen-receptive structures are the
ovules rather than the stigmatic portion of the carpels.
3.
Most gymnosperms lack vessels in their xylem (unlike flowering plants
which have both vessels and tracheids), except for
the gnetophytes, which have vessels.
Considering the relatively
small number of living gymnosperms (about 720 species in 65 genera), they are
remarkably diverse in their reproductive structures and leaf types.
Gymnosperms, like angiosperms
(the flowering plants), differ from seedless plants (like mosses and ferns) in
not requiring water for sperm to swim in to reach the egg. This means that the
movement of pollen (male gamete) to ovule (female gamete) in seed plants relies
on airborne transport, not water transport. Consequently, most gymnosperms
produce huge amounts of pollen.
1. In gymnosperms,
pollen is found (located) in stamen-like structures called strobili
(various types of cones). (strobilus = singular).
2. The pollen grains
of Pinus and several other genera have bladder-like wings.
3. Each male of a
pine tree cone annually releases an estimated 1-2 million pollen grains.
4. Pollination in
gymnosperms involves a pollination droplet that protrudes from the micropyle when pollen grains are being shed. The droplet
provides a large, sticky surface that catches the normally wind-borne pollen
grains of gymnosperms so that the ovule is more likely to be fertilized. After
pollination the droplet evaporates and contracts, carrying the pollen grains
into the pollen chamber and into contact with the ovule.
The most dramatic differences
between gymnosperms and other plants involve pollen and seeds and the organs
that bear them. These features differ significantly from those of comparable
organs of flowering plants.
Strobilus (cone) – the reproductive structure
in gymnosperms. In conifers, this
consists of an ovoid, cylindrical, or spherical cluster of sporophylls
(cone scales) arranged around a central axis. The place where
seeds of gymnosperms are produced; essentially, an evolutionarily modified
branch.
There are 4 major divisions of plants within the
gymnosperms:
1. Ginkgophyta (Ginkgo: maidenhair tree),
2. Cycadophyta (Cycads),
3. Gnetophyta (Gnetophytes), and
4. Pinophyta or Coniferophyta (the
conifers).
A. Ginkgo
1. Ginkgo biloba, or maidenhair fern because of the resemblance of its
fan-shaped leaves to those of maidenhair ferns, is the only living
representative of the division. It is the oldest known genus (and species) of
living trees. Fossil ginkgo leaves and wood have been discovered that date back
200 million years, and are nearly identical to those of the modern-day ginkgo.
2.
It is exclusively dioecious and deciduous.
3.
It has distinctive fan-shaped leaves with dichotomous venation; it is
deciduous. Both attributes resemble angiosperms. However, the fleshy seed coat
of Ginkgo can easily be mistaken for a fruit.
4.
The seeds of Ginkgo include a massive integument (outer coating of an ovule;
later becoming testa of a seed) that consists of a
fleshy outer layer, a hard, stony middle layer, and an inner layer that is dry
and papery.
a.
Mature seeds have
the size and appearance of small plums, but the fleshy integument has a
nauseating odor (like a vomitorium) and irritates the
skin of some people. (Nevertheless, pickled Ginkgo seeds are a delicacy in some
parts of
b.
Paired ovule, not
in cones, produced at the tips of short branches on the female trees.
5.
They are a popular cultivated tree, but it is apparently extinct in nature.
a. All living Ginkgo trees are descendants of plants
that were grown in temple gardens of
b. Most of the genetic diversity of Ginkgo has
probably been lost in cultivation, because most nurseries propagate only
cuttings of microsporangiate trees (male trees) to
avoid the stinky and messy seeds.
c. Introduced to the American continent and
d. Part of its popularity has to do with its putative
resistance to many types of air pollution common around industrial sites.
B. Cycads
(from the Greek kykas,
“a palm”). Cycadophyta
1.
Very important
plant group during the Triassic period, which occurred
aprox. 250 million years ago, and is sometimes
referred to as “the age of the cycads”.
2.
In the few
surviving cycads, there are about 10 genera and 100 spp., distributed primarily
in the tropical and subtropical regions of the world.
a.
There only 2 species of cycads native to the
3.
All spp. of
cycads are dioecious, producing microsporangial
(male) cones and ovulate (female) cones at the
terminal end of the trunk (which is unbranched).
4.
Cycads have
palm-like leaves that bear no resemblance to leaves of other living
gymnosperms.
a.
Under favorable
conditions, cycads usually produce one crown of leaves each year.
b.
The roots of some
cycads may grow at the surface of the soil and contain nitrogen-fixing cyanobacteria.
c.
Very slow
growing; in one case, a plant reached the height of 6’ only after 1,000 years
of growth.
5.
Seeds of cycads
are like those of Ginkgo, except the inner layer is soft instead of papery.
6.
Whereas Ginkgo
has rather loose microstrobili and single-seeded
stalks, and deciduous simple leaves, cycads have large strobili
and persistent palm-like leaves.
C. Conifers
(Coniferophyta or Pinophyta)
1.
The informal name
of the group, conifers, signifies plants that bear cones, even though other
members of the gymnosperms also include cone-bearing species.
2.
Members of the
genus Pinus, considered typical for the Pinophyta,
fare the most abundant trees in the
3.
In
4. Pines
a.
The genus Pinus,
by far the largest genus within the conifers, consists of about 100 spp.
b.
Pines actually
have 2 kinds of leaves. The more obvious leaf is the pine needle, a simple
leaf, which is produced singly or clustered in definite numbers, called
fascicles, of generally 2 to 5 needles
c.
A fascicle always
forms a cylinder or collar when the leaves are held together. (They are
actually short shoots that are surrounded at their base by small, nonphotosynthetic, scalelike
leaves that usually fall off after 1 year of growth).
d.
Needle-bearing
fascicles are shed a few at a time, usually every 2-5 years, so that any pine
tree, while appearing evergreen, has a complete change of leaves every 5 years
or so. (Bristlecone pines are an exception: their needles last an average of
25-30 years). All genera of Pinophyta are evergreen
except the larches, bald cypress (Taxodium distichum), and the dawn redwood (Metasequoia
glyptostroboides) which are deciduous.
e.
Pines produce 2
kinds of cones (strobili).
1.
Male cones,
usually 1 cm or less in length, are smaller than female cones, and generally
occur in dense clusters on the ends of the lower branches each spring.
2.
The more familiar
female cones, which are on the tree year-round, are usually located on the upper
branches on the pine tree, and bear seeds after sexual reproduction has
occurred. Female cones vary considerably in size among pine species: about 2”
in bristlecone pine to over 2’ in sugar pine.
f.
So, pines are monoecious; male and female cones are produced separately,
but on the same tree.
g.
Unlike other
gymnosperms, in which pollination, fertilization, and seed maturation occur
within the same year, the pines have an extended reproductive cycle of 14-20
months. (The period elapsed between the appearance of the ovulate
cone and the maturation of pine seeds in those cones. Fertilization takes place
about 1 year after pollination, and seed maturation takes several additional
months). Some seeds are shed as soon as they mature, whereas others remain
within the female cones for weeks, months, or even years before being shed.
h.
Other members of Pinophyta in the Northern Hemisphere also have narrow
leaves that have a small point at the tip, but they are not held together in a
fascicle like those in Pinus. These include yews (Taxus),
fir (Abies), larches (Larix),
redwoods (Sequoia
sempervirens), and spruces (Picea).
5.
Leaves of
Cypresses (Cupresses) and juniper (Juniperus) are scalelike at
maturity.
6.
Diversity within Pinophyta is reflected in the wide variety of reproductive
structures and variations of the reproductive cycle.
a.
Pines, firs,
Douglas-firs, spruces, etc bear cones (stobili) of
different sizes and shapes.
b.
Seed cones may be
absent entirely, as in the yews, which bears seeds surrounded by a fleshy,
cuplike covering that is an outgrowth from the base of the seed. The seed
covering is red and attracts birds, which eat them and disperse the seeds.
c.
Junipers have stobili, fleshy cones, with a berry-like appearance. The
fleshy scales are fused together and completely envelop the seed. Cones at
maturity may be orange, red, brown, purple, or black, depending on the species.
Birds eat the cones and swallow the seeds, which are then dispersed in their
droppings.
Pinaceae is monoecious; Cupressaceae is monoecious or dioecious.
7.
Vegetative
features of Pinaceae are important in their
ecological success.
a.
Most species have
a strongly dominant central axis and grow with a characteristic spire-like
habit, which helps to maximize photosynthesis when the solar angel is low, and
enables the tree to shed snow.
b.
The stems,
leaves, and cones have resin ducts that contain substances that seal damaged
surfaces and are somewhat insect-resistant.
c.
The needle-like
leaves have sunken stomata and a thick epidermis to retard water loss.
d.
Members of Pinaceae have small, ephemeral pollen cones and small to
large woody ovulate (seed) cones.
i.
A pollen cone
consists of a central axis and numerous small, flat, microsporophylls,
each with two microsporangia on its lower surface.
Scales of pollen cones remain small and soft. They are usually produced in
clusters near the tips of lower branches (ovulate cones are produced either
singly or in pairs on the younger branches). Within the same species the mature
pollen cones are always small, commonly less that 1” long, whereas the ovulate cones are large, measuring up to 2’ long in the
sugar pine. The pollen cone develops during early spring.
ii.
Ovulate cones are
more complicated. The cone has a woody axis to which are attached spirally
arranged cone scales (ovuliferous scales), each of
which is subtended by a scale-like bract. Two ovules are attached to the upper
surface of each scale. These mature in most members of the family as winged
seeds that break free from the cone scale at the time the woody cone scales are
dry and separate.
iii.
The transfer of
pollen from the pollen cone to the ovulate cone is
called pollination. After the release of pollen grains, the pollen cone dries
up and falls off. A pine tree produces a prodigious amount of pollen, most of
which never lands on the female cones, but instead settles on any stationary
object as a fine yellow powder.
iv.
Ovulate cones
usually remain on the tree for about 2 years.
v.
Pine seeds are
usually dispersed during autumn when the scales of the
ovulate cones separate.
vi.
Cones of some
pines so not open so gracefully or easily: these cones often require a fire to
cause them to open and release their seeds (serotiny).
The cones of some pines explode like popcorn when heated.
e.
Economically the Pinaceae is the most important gymnosperm family. Many
species are important lumber trees. In much of the world the Pinaceae is the principal source of wood pulp for paper
(white spruce, Picea glauca). Some
are economically valuable as sources of resins (sticky, aromatic substance in
resin canals of conifers), turpentine (volatile liquid used in solvents as
paint thinner and varnishes), rosin (used in the manufacture of floor waxes and
paper coating; on bows of stringed instruments, and baseball pitchers use it
for better grip on the ball(, amber (hard, yellowish
to brown translucent substance, is fossilized resin, used chiefly in
jewelry),and other extracts.
i.
Douglas fir and
Hemlock/Douglas fir mix used in construction. Strong wood, relatively free of
knots as a result of rapid growth with less branching than other conifers.
ii.
Redwoods (Sequoia sempervirens)
are prized for their wood, which contains substances that inhibit the growth of
fungi and bacteria.
iii.
Spruce wood is
important to the music industry. Tracheids of spruces
have spiral thickenings on the inner walls, which apparently give the wood a
resonance that makes it ideal for use as soundboards in violins and guitars.
D. Gnetophytes
1.
This group
includes some of the most distinctive, and bizarre, seed plants.
2.
There are 3
clearly defined, morphologically dissimilar genera of these gymnosperms: Ephedra (40 spp.), Gnetum (30 spp.), and Welwitschia
(1 sp).
3.
One feature they
all share, in addition to being gymnosperms, is the presence of vessel elements
in their wood, a cell type absent in other gymnosperms.
4.
Ephedra
(Mormon tea, ma huang, joint fir – the first two
names referring to its use as a stimulating or medicinal tea. Ma huang is an Asian species, E. sinica, which contains chemicals that
are similar to those of human neurotransmitters, like ephedrine, a drug used
for the relief of allergic symptoms because of its ability to constrict blood
vessels. It is also a heart stimulant and hass been
sold over-the-counter in weight-c0ntrol medications and herbal energy-boosters.
(In 1996 at least 15 deaths were attributed to chronic use or overdose of
products containing Ephedra, as were the two athletes who recently died). Joint
fir refers to the leafless appearance of Ephedra stems, which resemble the
jointed stems of Equisetum, horsetail.)
a.
Ephedra is not
actually leafless; its leaves are small and lose their photosynthetic
capability as they mature. Most photosynthesis in Ephedra occurs in green
stems.
b.
They are
restricted to desert or semi-desert habitats, the arid southwest in the
5.
Ephedra are either
monoecious or dioecious.
6.
Most spp of Gnetum are tropical and dioecious,
either climbing vines of trees, all with broad leaves similar to those of woody
dicots.
7.
Welwitschia mirabilis is the sole living representative of its
genus and looks more like something out of a science fiction movie than a real
plant.
c.
This slow-growing
species is confined to the
d.
Most of its
moisture is derived from fog that rolls in from the ocean at night.
e.
The woody stem is
concave and bark-encrusted, maybe as much as 1.5 meters in diameter, and
connected to a very large taproot. Much of the stem is underground.
f.
Mature plants
have a pair of strap-shaped leaves (up to 9’ long), which persist throughout
the life of the plant. Each leaf has a meristem at
its base, which constantly replaces tissue that is lost at its drier, aging
tip.
g.
As the plant
ages, the leaves periodically split lengthwise giving the plant the appearance
of having many leaves.
h.
Welwitschia is dioecious, producing
male and female strobili in different plants,
originating from the axils (angle formed between upper side of petiole and
stem) of leaves.
i.
Growth of these
plants is unusually slow die to the dry environment.
Gnetophytes are the most distinctive of gymnosperms because of their
similarities with angiosperms.
a.
They have
flower-like compound strobili.
b.
They have vessels
in secondary xylem.
c.
They have double
fertilization. (However, this double fertilization does not result in the
formation of endosperm. Instead, the diploid cell from fertilization by the
second sperm disintegrates).