THE PLANT KINGDOM
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CHARACTERISTICS OF PLANTS:
Plants are eukaryotic, multi-celled photoautotrophs.
Plants develop from embryos.
Embryos are multi-celled structures surrounded by maternal tissue. They develop
from an egg cell fertilized by a sperm cell (or a sperm cell modified as a
pollen nucleus). So, plants are capable of sexual reproduction and their embryos
are composed of diploid cells. However, many plants also grow and reproduce
asexually, and a few have lost the ability to have sex.
NOTE: Speaking of sex, because Linnaeus used the number and kind of sex organs
of flowers as important characteristics in classifying plants, his book was not
considered suitable reading for young ladies at the time his work was published
(mid-eighteenth century).
Plants contain chlorophyll a and
b; their cell walls are composed of
cellulose; they store energy in the form of starch, and their sperm have two
forward-directed flagella.
Plants share this set of features with the green algae, their most probable
ancestors. Other groups of algae are quite different. For example, red algae
contain chlorophyll a and
d and never have flagella; brown
algae contain chlorophyll a and
c and store energy as the
carbohydrate laminarin; euglenids lack a cellulose cell wall and are completely
asexual. The fact that so many plants maintain mycorrhizal relationships with
fungi is good evidence that these symbiotic partnerships arose very early in
plant evolution.
At least 295,000 species of plants exist today. Most are vascular plants, that
is, they have internal tissues specialized for transporting water and dissolved
substances (e.g., nutrients and minerals) through the plant body.
In leaving the aquatic environment, the ancestors of plants had to adapt to very
different conditions that required a whole set of new adaptations:
Air is much less dense than water.
Land plants had to develop support against gravity, including strengthened cell
walls and stems. Many deposit lignin, the main component of wood, which
permitted increased size and branching.
Nutrients and water must be extracted from the soil rather that from the
surrounding aquatic medium.
Simple land plants possess simple rootlike rhizoids. The great majority of
plants bear roots that not only extract nutrients and water from the soil, but
transport it via vessels up to the rest of the plant.
Water loss must be prevented on land.
Many plants developed a waxy surface cuticle to reduce water loss.
Stomata
are tiny openings on plant surfaces (mostly leaves) that balance the often
contradictory needs of a plant for reducing water loss but maintaining CO2
intake.
Growth toward the light requires increased and elevated surface areas for
photosynthesis and CO2 absorption, and for outcompeting neighbors.
The increased distance between nutrient/water absorbing areas in the soil and
light-collecting areas raised above the ground created strong selection pressure
for the development of vascular tissue--cells that could transport materials
through the plant body.
Xylem vessels distribute water and dissolved ions.
Phloem tissues distribute sugars and other products of photosynthesis
The development of such transport tissues permitted plants to grow larger and
develop specialized structures such as leaves, stems and roots.
With increasing adaptation to drier terrestrial habitats, the haploid phase of
the plant life cycle diminished and the diploid stage became dominant.
The development of two different kinds of spores led to the evolution of pollen
and seeds (see below).
The
mosses
and their relatives (liverworts and hornworts) are small plants, at most 20 cm
tall, that typically grow in continuously or seasonally moist habitats. The have
simple rootlike, leaflike and stemlike parts, but they lack xylem and phloem
tissues (i.e., they are nonvascular). Their small size and preference for moist
habitats permit the distribution of nutrients and water through their bodies
without special transport tissues. Over 18,000 species of bryophytes exist.
Unlike all other living plants, the haploid gametophyte phase dominates the life
cycle. Moss and liverwort "plants" are gametophytes made up of haploid cells.
Male gametophytes produce sperm that must swim through at least a thin dewy film
of water to reach the eggs borne by the female gametophytes (hence, the
restriction to moist habitats). The fertilized egg develops into a diploid
sporophyte that is nothing more than a tall slender stalk topped by a spore
chamber. It remains dependent upon the gametophyte plant that bears it. Like
lichens, many mosses can dry out and revive when moisture returns.
Peat mosses (Sphagnum), unlike most
slow-growing mosses, can grow fast enough to produce more organic material than
an equivalent acreage of corn. They form moist organic carpets in cooler regions
and can soak up five times as much moisture as cotton. Because they produce
acids that reduce the activity of bacterial and fungal decomposers, their
remains may accumulate in moist low-lying areas as thick, dense, compressed mats
called peat bogs. Such peat can be harvested and dried for fuel. It is also used
in gardening to increase the soil's water-holding capacity. Harvesting peat from
deeper bogs occasionally turns up well-preserved corpses or the bodies of
late-Ice Age mammals such as woolly rhinoceroses.
Land plants with vascular tissue arose over 400 million years ago. Some of their
more direct descendants still survive. Like the mosses, their sperm cells still
require a water medium in order to reach the egg, so they remain tied to humid
environments. Unlike the mosses and other bryophytes, however, they have true
vascular tissues and the sporophyte grows independently of the gametophyte; it
is typically the larger, longer-lived phase of the life cycle.
Whisk ferns (psilophytes) have branching photosynthetic stems, but neither roots
nor leaves. They are at least superficially similar to fossils of some of the
first land plants such as
Cooksonia
from the Silurian Period. However, they are probably more closely related to
ferns.
Club mosses (lycophytes) are not mosses, but small plants with true leaves,
roots and stems (although the leaves are quite small). Although unobtrusive,
they can be found from the tropics to the Arctic, often forming moss-like mats
on damp forest floors. Cone-shaped leaf clusters contain spore sacs. One genus
has small and large spores, a harbinger of the distinction between tiny male
pollen and large female eggs that will appear in more advanced plant groups.
Some 350 million years ago, giant tree-sized club mosses reached enormous
heights and often dominated vast swamp forests. Their remains are major
contributors to modern coal deposits.
Horsetails (sphenophytes) of great size also contributed to ancient swamp
forests (and coal deposits). Although their modern descendants reach far more
modest dimensions, some, such as
Equisetum, have remained essentially unchanged for about 300 million years.
Their upright, hollow, pencil-like stems with whorls of slender or scale-like
leaves commonly occur in disrupted habitats such as muddy stream banks,
roadsides, railroad beds and vacant lots. While horsetail sporophytes may reach
several feet tall, the separate gametophyte is an inconspicuous one to ten
millimeters across. Pioneers took advantage of the gritty texture of horsetails
(due to silica deposits in their stems), using them as pot scrubbers and calling
them scouring rushes.
Ferns (pteridophytes) are the best known and most diverse (some 12,000 species)
of seedless plants. The vast majority live in the tropics, but people keep them
as houseplants around the world. Ferns range from tiny floating species no more
than 1 cm across to giant rain forest tree ferns that reach 25 m tall.
Some of the tiny floating species have a symbiotic relationship with
nitrogen-fixing cyanobacteria and can thus supply nitrogen to the rice paddies
where they grow. Regardless of their size, ferns remain tied to humid
conditions; their sperm still must swim through water to the egg, even though
the water may be no more than a thin film of dew.
As with the other seedless vascular plants, the fern life cycle is split between
haploid gametophyte and diploid sporophyte phases. In ferns, however, the
sporophyte, which is the familiar fern plant, dominates the life cycle. Thick
horizontal underground stems, called rhizomes, produce both roots and leaves,
called fronds. Of all of the living seedless vascular plants, ferns are the only
ones with large complex leaves. Haploid spores develop on the undersides of fern
fronds in little brownish, regularly-spaced dots called sori (singular: sorus).
When mature, the sori rupture and the spores drift off on the wind. A settling
spore germinates and develops into a tiny, green heart-shaped gametophyte that
produces both eggs and sperm. The fertilized egg then grows into a new familiar
fern sporophyte.
All of the remaining plant groups discussed below bear seeds. None require an
aquatic medium for fertilization. They thus became free to colonize higher and
drier habitats. In the living representatives of these plants, the haploid
gametophyte phase has been greatly reduced. All of the seed-bearing plants that
one sees are sporophytes. They develop modified leaves or leaflike structures
(including pinecones and flowers) that produce spores: tiny male microspores and
large female megaspores. However, unlike the situation in ferns and the other
seedless plants, the spores are not shed. They develop into gametophytes which
are nothing more than small clusters of cells that develop on or in the
sporophyte, and are dependent upon the sporophyte for nutrition.
Ovules ("little egg") consist of the female gametophyte (a small cluster of
cells including one or two egg cells and surrounding nutritive material) plus an
outer coat contributed by the parent sporophyte plant.
Pollen grains are the microscopic male gametophytes. They develop from the
microspores and contain one or two haploid sperm nuclei inside a tough outer
coat. Pollen may be distributed by the wind, an insect or other animal
pollinator (e.g., bird, bat, plant breeder). When a pollen grain lands on the
right surface of a sporophyte (the same plant or another individual of the same
species), near a female gametophyte, it produces a fine pollen tube that grows
through the sporophyte tissue. When the pollen tube reaches an ovule, one or two
sperm nuclei are released through the tube and fertilization takes place.
A seed consists of a many-celled embryo sporophyte (that develops from the
fertilized [diploid] ovule) surrounded by nutritive food reserves and a
protective outer seed coat (contributed by the parent sporophyte). The young
embryo goes through a dormant, or resting, stage, during which the seed coat
protects it from drying out or from predators. Many seeds have adaptations that
promote their dispersal, either by wind or by animals (e.g., the outer layers of
the seed coat may taste good). As the embryo develops, it is nourished by the
food reserves that surround it. The combination of resistant pollen and seeds,
and the ease with which they can be spread, are major reasons why seed plants
have so successfully populated so many habitats around the world.
Gymnosperm means "naked seed"; the group includes seed plants in which the seeds
lie exposed on surface of spore-producing structures. The first fossil evidence
for early gymnosperms (or progymnosperms) appears in rocks of Devonian age.
Early progymnosperms combined fern-like features (e.g., perhaps
moisture-dependent reproductive cycles) with gymnosperm structure, but
eventually developed seeds. Fossils of the extinct "seed fern",
Glossopteris, which are found in
South America, Africa, India, Australia and Antarctica, provided early evidence
for Pangaea and continental drift. Conifers dominated all forests for much of
the Mesozoic Era until they were displaced from supremacy in many habitats
during the Cretaceous Period by flowering plants.
CONIFERS
This group includes the pines, spruces, firs, hemlocks, cypresses, yews,
junipers, redwoods, sequioas and larchs. Most are trees, but some are smaller
shrubs. About 550 species occur today, chiefly in cold temperate forests and
alpine habitats (although cypresses are abundant in southern swamps such as the
Everglades). Conifers get their name from their cones, which are clusters of
modified leaves that enclose the spore-producing structures. Pollen-producing
male cones and ovule/seed-producing female cones appear quite different. Pollen
grains are distributed only by the wind, typically in the spring. When a pollen
grain lands on a female cone, a pollen tube develops to transport the sperm
nucleus to the ovule, but fertilization may only occur months or even a year
after pollination.
Conifers bear tough, waxy, needle- or scale-like leaves. Although they shed some
leaves throughout the year, they usually retain enough to warrant the name
"evergreen". They include some of the largest and oldest of living things.
Sequoias are the most massive of plants and some living bristlecone pines are
more than 4000 years old. Araucaria,
the Norfolk Island pine and its close relatives, are living fossils. Their wood
is virtually identical to that of fossil logs from the 235-million year old
Petrified Forest of Arizona.
Conifers are economically important in many ways. Their woods are major sources
of lumber for building, and pulpwood for paper. Sap-like secretions called
resins are used to produce turpentine, varnishes, printing ink, plastics and
pitch. Solidified fossil resin forms the semiprecious stone, amber. Pine nuts
(seeds) are edible. Taxol, a product of yew trees, shows promise as an
anti-cancer agent.
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CYCADS
Cycads are chiefly tropical plants that typically have thick stout trunks topped
with palm-like fronds. Massive cones develop on separate male and female plants.
Although wind- or insect-borne pollen produces a pollen tube when it lands on a
female cone, the sperm still has flagella and swims a short distance to the
ovule (though it is not exposed). Cycads were abundant and diverse during the
Mesozoic, but only about 100 slow-growing species survive today. Many produce
toxic alkaloids.
GINKGOS
A single species, Ginkgo biloba, the
maidenhair tree, survives from the Mesozoic and is now only know from cultivated
plants. Male trees are resistant to insects and pollution and are thus widely
planted in urban environments. However, the seeds of female plants have a fleshy
coat that produces an unpleasant stench when ripe. The fan-shaped leaves are
easily recognizable. Extracts supposedly have antioxidant properties that,
according to some, improve memory function.
