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Chapter 23: Seed Plants II: Angiosperms (Monocots (Commelinoid Monocots…
Chapter 23: Seed Plants II: Angiosperms
Concepts
Embryophytes became distinct from charophyte green algae.
Since then, the flowering plant clade--the angiosperms--contains the greatest number living species--257,000.
All classified in a single division, the
Magnoliophyta
(often called the Angiospermophyta).
#
In most
angiosperm carpels
, the edges of sporophyll primordia crowd against each other and grow shut.
Sometimes leaves a visible suture, sometimes closing so completely that no sign of a seam remains.
This is called a
closed carpel
.
It develops into a fruit that encloses the embryos as they develop into seeds.
"Angio" means "clothed"
In the transition from their ancestral group to being angiosperms, fertilization evolved.
Such that the second sperm cell of the pollen tube fuses with the polar nuclei of the megagmetophyte, producing the endosperm nucleus.
This process of
double fertilization
is universal in flowering plants.
Within the vegetative body, the major transitions were the evolution of vessel elements and sieve tubes.
Leaves became...
developed reticulate venation
more polymorphic
broader
adaptable to a variety of functions, not just photosynthesis
Quite a few plants were considered to be basal angiosperms on the basis of having wood without vessels.
It was thought that these were
primitively vesselless
(that they lacked vessels because their ancestors lacked them).
Many other features, however, were inconsistent with the idea of these being a natural, early group.
Current hypothesis is that perhaps only
Amborella
is primitively vesselless and that the others arose after vessels had originated but then these groups lost them.
If so, they are
secondarily vesselless.
Their tracheid-based, gymnosperm-like wood is a derived feature that looks like a primitive one and misled us for years.
Sieve tubes may have originated next.
Several species still have sieve cells in their phloem.
All gymnosperms and seed ferns are or were woody plants and so are most of the basal angiosperms and eudicots
1 more item...
Other derived features are...
Fusion of petals into one structure (
sympetally
).
Floral
zygomorphy
, that is, flowers that are bilaterally symmetrical, not radially symmetrical.
The fusion of the carpels into a single structure (a
pistil
).
Even more specialized characters such as succulence, parasitism, epiphytism, bulbs, corms, tubers, tendrils, and insect-trapping leaves.
Originated only much later, after angiosperms diversified into many clades.
These innovations present in only a few families or species.
It should not be thought that the flowering plants are now "finished," that all their evolution has already happened.
Eudicots
Rosid Clade
Named for the rose order Rosales
Consists of many families that, taken as a whole, are so diverse with respect to vegetative body, flowers, chemistry, and ecology
Makes it difficult to see they are all related.
Some share enough characters with others to indicate a relationship
Those of the second group share different features with a third group and so on.
Leads to a distinguishable, larger picture of phylogenetic relationships
Consist of several small orders
One of the small orders: Vitales contains Vitaceae, the grape family.
One small order: Geraniales, contains the geranium family, Geraniaceae.
Also consist of two large groups.
Fabids
(
eurosids I
)
Malvids
(
eurosids II
)
Contain more than 100 families; difficult to find universal characters
More derived than basal tricolates
None of them have any of the highly relictual features found in basal angiosperms.
Pinnately compound leaves.
14 large orders with over 50,000 species
Asterid Clade
Most derived large clade
Contains plants such as sunflower, periwinkle, petunia, and morning glory.
A sister to clade of rosids, originated perhaps as recently as 60 million years ago
Even its most basal members were much more highly derived than plants in the basal angiosperms.
Can be distinguished from other angiosperms on the basis of 3 features:
Sympetalous flowers
Always have just a few stamens, not more than the # of petal lobes.
Stamens alternate with petals.
Basal Eudicots
Water lilies
Ranunculales and several others are believed to be clades that diverged at early stages in eudicot evolution.
Ranunculales
Flowers have so little fusion of parts
Each has usually many stamens and carpels
All remain separate of others
Stamens remain free or may fuse together in a tube or fuse to carpels.
Familiar members
Windflower (
Anemone
)
Clematis
Buttercups
Poppy family, Papaveraceae
Numerous ornamental species
Eschscholzia
(California poppy)
Papaver
(poppies)
Argemone (pickly poppy)
Hamamelid families
Large trees with reduced wind-pollinated flowers
Usually in dangling inflorescences containing many staminate flowers and just a few carpellate flowers.
Ex: Plantanaceae, sycamores (aka plane trees)
Have small, inconspicuous clusters of flowers in the spring
Flowers mature into spherical clusters of hundreds of tiny dry fruits
Caryophyllales
DNA-based combinations
Core group
four-o'clocks (Nyctaginaceae)
spinach, beets, and Russian thistle
carnations and chickweeds
bougainvillea
portulaca (Portulacaceae)
iceplant (Aizoaceae)
cacti (Cactaceae)
Other flowering plants have
anthocyanin pigments
in their flowers, almost all Caryophyllales instead produce a group of water-soluble pigments called
betalains
.
Endosperm develops only a little and then fails to continue growing.
Instead, nuclleus cells proliferate and form a nutritive tissue called
perisperm,
which surrounds the developing embryo.
Perisperm usually absorbed almost completely by the time the seed is mature.
Nature of sieve tube plastids.
Postulated to have arisen from ancestors similar to Ranunculaceae
#
Santalales
A small order of highly modified plants, most of which are parasitic.
Sandalwood family (Santalaceae) contains the large tree Santalum from which sandalwood incense is obtained
Pollen grains have either 3 germination spores (
tricolpate
) or have some condition derived from the tricolpate mechanism.
Changing Concepts About Early Angiosperms
Early angiosperm concepts change as our knowledge of existing and fossil plants has become more complete.
Examples:
In last century, wind-pollinated trees (alders, elms, oaks, and plane trees) were grouped in "subclass Hamamelidae."
#
Considered most relictual living flowering plants.
These species tend to be large trees with dense wood.
Flowers small and simple.
Usually without sepals and petals.
Many gymnosperms have similar features.
HOWEVER, could NOT be relictual because wood contains...
abundant parenchyma
fibers
vessels
features not found in gymnosperms
Simple through specialization and reduction:
Wind-pollinated flowers do not need to attract pollinators.
They do not need to be large or colorful-they do not need petals or sepals.
Recent DNA studies
Indicate that syndrome of large, wind-pollinated trees is a derived condition within angiosperms
1 more item...
Approximately 100 years ago
C.E. Bessey developed hypothesis of
ranalean flower
A
Magnolia
-type flower was thought to be relictual.
#
Such a flower is
generalized
That is, has all parts (sepals, petals, stamens, and carpels)
And arranged spirally
Carpels occur in a superior position, above the other parts
Easy to postulate evolution of all various existing flower types from a ranalean, generalized ancestor.
Example
4 more items...
Most botanists long ago concluded that angiosperms are monophyletic.
Complex features probably did not evolve more than once.
Flowers
Developmental plasticity
Double fertilization
Almost all recent DNA studies indicate monophyly of angiosperms.
Gymnosperms of Jurassic and Triassic Periods
Focus on centering on cycadophytes and glossopterids.
Much of the various groups have begun to develop angiosperm-like features.
Monocots
Commelinoid Monocots
Differ from others in several unusual synapomorphies.
Ultraviolet-fluorescent compounds
Pollen contains starch, as does their endosperm
Walls unusual types of hemicelluloses
Multiple studies of
rbcl
,
atpB
, and other DNA regions support this as a distinct clade.
Unique types of epicuticular wax.
Arecales
Contains familiar palms, in family Arecaceae
About 3,500 species, all of which are easily recognizable by their solitary trunk
Varies from 1.0 m wide in some and only 1.0 cm in others.
All have scattered vascular bundles
Leaves occur near the shoot apex
Never distributed along the length of the stem.
Simple leaves, after fully expanded torn by wind
Either into a pinnate pattern (feather palms)
Or palmate ones (fan palms)
Poales
Contains the grass family Poaceae, cattails (
Typha
), bromeliads, and sedges.
Contains about 8,000 species
Include most food like wheat, barley, oats, rye, corn, rice, bamboo, and sugar cane.
50% of all calories consumed come from grass seeds
Main sources of meat--cow, pig, and sheep--raised on grassland and fed corn.
Grasses abundant in flat, open, and dry regions in the central areas of all continents.
Accounts for one quarter of all vegetated land on Earth.
Commelinales
Perennial herb
Roots thick, fibrous
Shoots prostrate to decumbent
Sometimes rooting at the nodes and often forming mats, or ascending or straggling
Usually much branched, with a line of pubescence or glabrous
Often tinged with red
Leaves distichous
Zingiberales
Contains familiar houseplants
Maranta, Calathea
, canna lilies (
Canna
), and gingers (
Zingiber, Hedychium
)
In warmer southern states, banana (
Musa
) and bird-of-paradise (
Strelitzia
)
Tend to have large, showy flowers pollinated by insects, birds, or bats.
Many flowers have derived features
Adjacent sepals are often fused to each other
Forming a tube
Same is often true of the petals
Bilaterally symmetrical
Gynoecium
Inferior, located below the sepals and petals
Typically consists of 3 carpels that have fused almost completely.
Approx. 1,000 species
Almost all tropical, most soft, non-leathery herbs.
Leaves broad, and have a petiole.
Plants grow in heavy shade of jungle understory where light interception is important.
Widely believed to have arisen from early angiosperms approx. 80 to 100 million, perhaps even 120 million years ago.
All lack ordinary secondary growth and wood.
Ancestors probably herbs with either no vascular cambium or little cambial activity.
Gynoecia of many monocots composed of several carpels (usually 3)
Either free of each other or at most only slightly fused together.
The perianth usually consists of 3 outer and 3 inner members.
Look so similar that instead of referring to as sepals and petals, known as
tepals
.
Classification of Flowering Plants
In the 1980s and 1990s, the most widely used monograph of the entire division was
An Integrated System of Classification of Flowering Plants
By Dr. Arthur Cronquist of the New York Botanical Garden
Since then, cladistic studies involving DNA, biochemistry, and anatomy often proposed phylogenies that agreed well with those of Dr. Cronquist for certain clades but disagreed with others.
Two widely used reference books are...
Plant Systematics, A Phylogenetic Approach
(3rd Edition) by W.S. Judd et al.
Plant Systematics
by Michael Simpson
Angiosperm Phylogeny Group (APG), which is a semi-informal collaboration of many systematists
Classifications on www.mobot.org/MOBOT/research/APweb/
Soon after origin, flowering plants began to follow two distinct lines of evolution.
monocots
: informal term for any member of the flowering plant class Liliopsida
Ex: lily, iris, palm, agave
eudicots
: the clade of angiosperms that contains most species formerly known as dicots
Monocot/eudicot divergence did not occur right away,
Instead, the early angiosperms diverged into several clades now called the
basal angiosperms
.
Not newly discovered
Classified as monocots or dicots before
Many different DNA sequences indicate they had become reproductively separate from the other angiosperms very early.
Amborella
and
Austrobaileyales
greatly resemble eudicots.
#
Cannot be classified with eudicot clade
The common ancestor that includes both them and the eudicots also includes monocots.
Cannot construct a group that includes some descendants of a common ancestor but excludes others.
Nymphaeales (water lilies) have many features of monocots, but classifying them as monocots create a polyphyletic group.
Basal Angiosperms
#
#
Contain living descendants of several groups that originated while angiosperms were still a young clade.
Ancestors became reproductively isolated before distinctive angiosperm traits had originated.
3 groups of extant descendants of these clades:
Nymphaeaceae
Water lilies
#
Lack any wood
Small, soft-bodied herbs
Stems must be submerged underwater
Vascular bundles scattered
Exposure to any conditions on land result in death
Large colorful flowers
Pollinated by animals
Almost nothing in common with either gymnosperms or seed ferns
Austrobaileyales
Contains woody trees with bisexual flowers
Stamens and carpels similar to
Amborella
Differ against
Amborella
in so many features, cannot resemble the earliest angiosperms
#
Instead, one or the other--or all three--have undergone considerable evolutionary change since their clades originated.
Amborellaceae
Only one species in clade
Very different, being small trees in forests of New Caledonia
Wood contains tracheids
Wood has no vessels and little parenchyma
Dioecious
Some plants have staminate flowers
Have numerous stamens
Flowers are small with 5 or 11 tepals
Others carpellate flowers
Have 5 or 6 carpels whose edges do not seal tightly
Have interlocking, secretory hairs that act as stigma
Have not remained static evolutionarily and have not perserved all ancestral features.
Magnoliid Clade
Ancestors diverged from early ancestors than clades above.
Based on DNA evidence
Morphologically, the magnoliids do not differ greatly from these two earlier clades
Ex: Family Magnoliaceae contains trees with wood similar to that of gymnosperms in that it lacks vessels, fibers, and axial parenchyma.
Magnolia
flowers have numerous stamens and carpels arranged in spirals
Carpels not fused together into a pistil as occurs in almost all monocots and eudicots.
Pollen grains have only a single germination pore.
They are
uniaperturate
, as are all other basal angiosperms and monocots.
Eudicots have 3 germination pores.
Other magnoliids are laurels and avocado (Laurales) and peppers and peperomias (Piperales).
