Seed Plants ll: Angiosperms (Concepts (Angiosperm Carpels # (the edges…
Seed Plants ll: Angiosperms
all classified together in a single division, the Magnoliophyta
the flowering plant clade -the angiosperms-contains the greatest number living species-257,000
as angiosperms evolved to provide reward for pollinators, natural selection also was giving them numerous new types compounds
wood of almost all angiosperms is more complex that that of an conifer
the earliest fossils clearly recognizable as parts of flowering plants are preserved pollen grains in rocks more than 130 million years old.
the first angiosperms were probably small trees or shrubs because plants of this type survive well in drier regions
and because the most likely ancestors -other seed plants were woody trees
the evolutionary process from gymnosperms to flowering plants was not an easy process.
the edges of sporophyll primordia crowd against each other and grow shut , sometimes leaving a visible suture, sometimes
closing so completely that no sign of seam remains
and it develops into a fruit that encloses the embryos as they develop into seeds
in the transition from ancestral group to being angiosperms fertilization evolved such that the second sperm cell of the
pollen tube fuses with the polar nuclei of the megagametophyte, producing the endosperm nucleus
this process is called double fertilization and it6s universal in flowering plants
Structural modifications are most easily seen and studied, but probably a more fundamental transition was the acquisition of developmental plasticity
the capacity to survive mutations that alter growth and development.
the evolutionary changes involved in conversion of gymnospermsous line into an angiosperm line did not occur instantaneously
nor did they occur in all species
Quite a few plants were considered to be basal angiosperms on the basis of having wood without vessels, it was thought that these were
that they lacked vessels becauses their ancestors lacked them
other derived features are fusion of carpels into a single structure (
the fusion of petals into one structure (
) and floral
that is flowers that are bilaterally symmetrical not radially symmetrical
changing Concepts About Early Angiosperms
Concepts about the nature of early angiosperms have changed as our knowledge of existing and fossil plants has become more complete
in the last century, wind-pollinated trees were grouped together in a "subclass Hamamelidae" and were considered the most relictual living flowering plants
this is reasonable because these species tend to be large trees with dense wood, their flowers are small and
simple, usually without sepals and petals and many gymnosperms have similar feature
wind pollinated flowers do not need to attract pollinators, and thus they do not need to be large or colorful- they so not need petals or sepals.
recent DNA studies indicate that this syndrome of large wind pollinated trees is derived condition within angiosperms
and furthermore, it has evolved several times in various clades
Approx. 100 years ago, C.E. Bessy developed the hypothesis of the
, in which a
type flowering was thought to be relictual
such a flower is
that is it has all parts (sepals, petals. stamens and carpel, and these are arranged spirally.
also carpels occur in a superior position above the other parts.
most botanists had long ago concluded that angiosperms are monophyletic.
features as complex as double fertilization, flowers and developmental plasticity probably did not evolve more than once
almost all recent DNA studies also indicate monophyly of angiosperms
Currently many scientist believe that the transition from gymnosperms to angiosperms occurred during the Jurassic and lower Cretaceous periods of the Mesozoic Era
the earliest leaf fossil definitely considered to be those angiosperms are from the Lower Cretaceous period Apporx 130 million years ago
they represent both eudicots and monocots
Classification of Flowering plants
Magnoliophyta is such a large group with so many families, genera and species that it is rare for individual taxonomist to attempt to study and classify the entire group.
in the 1980s and 90s the most widely used monograph of the entire division was
An Integrated System Of Classification
of Flowering plants y Dr. Arthur Cronquist
of the New York Botanical Garden.
Since then cladisitic studies involving DNA, biochemistry, and anatomy often proposed phylogenies that agreed well with those of Dr. C for certain clades
but disagreed for others.
the areas where the two systems were inconsistent became the topics of more intense scrutiny of all characters
with subsequent adjustments in our concepts of phylogeny and classification
Soon after their origin , flowering plants began to fill two distinct lines of evolution and currently almost all angiosperms are classified
Any of a class of angiosperm plants having a single cotyledon in the seed. Monocotyledons have leaves with parallel veins, flower parts in multiples of three, and fibrous root systems. Their primary vascular bundles are scattered throughout the stem
An angiosperm having two cotyledons in the seed, leaves with a network of veins radiating from a central main vein, flower parts in multiples of four or five, and a ring of vascular cambium in the stem.
The basal angiosperms are the flowering plants which diverged from the lineage leading to most flowering plants.
currrently monocots are believed to arised from early angiosperms
approximately 80 to 100 million perhaps even 120 million years ago
because all monocots lack ordinary secondary growth and wood their ancestors were probably herbs either with no V.C. or little cambial activity
the gyneocia of many monocots are composed of several carpels that are either free of each other or at most only slightly fused together
the perianth usually consists of three outer and three inner members they often look so similar that rather than using the terms sepals and petals, the membrane are just called
rarely fuse to each other to form a tube instead they remain free.
these features must have been present in the group from which the monocots evloved
one theory about parallel venation of monocots leaves postulates that the ancestors had broad leaves and lived as aquatic plants
Early monocots diverged into species of clades whose extant membranes are classified into approx. ten orders
contains some aquatic herbs such as
an many other plants
these plants are most often found in swamps and marshes partly or entirely submerged.
many species retain a large number of plesiomorphic feature, others have become highly modified in response to the unusual selection pressures associated with the aquatic habitat
they have no transporation and thus mutations that results in the loss of stomata are not selecively disadvantageous
air chamber make the plants buoyant; therefore mutations that prevent the formation of lignified fibers are selectively advantageous
the plants do not waste carbon and energy by producing fibers that are needed for support.
the remaining eudicots are members of two very large very diverse clades, the rosids and asterids
the clade consists of many families that taken as a whole are so diverse with respect to vegetative body, flowers, chemistry and ecology that it is difficult to see they are related
however some share enough characters with others to indicate a relationship and those of the second group share different features with a third group
consists of several small orders and two large groups
large the fabids and malvids
interesting in that none of them has any of the highly relictula features foubd in many basal angiosperms