Green algae called charophytes
and plants probably evolved from a common ancestor. This theory was
based upon the following homologies between plants and the charophytes:
- Chloroplast structure.
The pigments beta- carotene and chlorophyll b are fund in both
organisms. The DNA of both organisms match closely,and the thylakoid
structures are stacked as grana.
- Biochemical similarity.
The cell wall is composed of cellulose. The charophytes cell wall
contains between 20 % and 26% cellulose. The peroxisomes of the
charophytes contain the same enzymes as that of the plants. Other
algae do not have this relationship.
- Similarities in mitosis.
The nuclear membrane is completely gone by late prophase, the spindle
fibers remain until cytokinesis, and cell plate formation involves the
cooperation of microtubules, actin, microfilaments, and vesicles.
- Gamete structure. The
sperm's ultrastructure of both plant and charophytes are very similar.
- Genetic relationship.
DNA and rRNA are very similar in both organisms.
- Plant as well as animal
life developed in the water. Plants must have had to gradually develop
characteristics that allowed them to survive on land. Life in shallow
water could have been part of a logical progression in the development
of adaptations for this life on land. Natural selection may have
favored shallow-water plants tolerant to periodic drying. Some of
these adaptations are: waxy cuticle, protection for gametes,and
protection for developing embryos. As the plants emerged from the
water they entered a new environment of direct sunlight, soil rich in
nutrients, and a predator free environment.
- Nonvascular Plants:
are plants that lack vascular tissue and require environmental water
to reproduce. The first three Divisions in the above chart are
nonvascular plants. General characteristics of this group include: a
waxy cuticle to slow down water loss and gametangia that
protect developing gametes. The male gamatangium is called the antheridium
which produces flagellated sperm. The female archegonium
produces a single egg. Bryophytes need water to reproduce. They do not
contain any vascular tissue for up right support. These plants usually
form mats of horizontally growing tissue.
Nonvascular plants that form spongy mats. The plant grips the ground
with elongated cells called rhizoids. Photosynthesis occurs in
the stem-like structures above the matted plant bodies. They undergo
an alternation of generations with the haploid generation being the
dominant form of this cycle. The sporophytes are generally smaller and
depend on the gametophyte for water and protection.
Plants with bodies divided into lobes, life cycle similar to that of
the mosses with special structures that propel the spores out of the
capsule, and the ability to reproduce asexually from structures called
gemmae. These are small bundles of cells that can bounce out of
cups on the surface of the gametophyte when hit by rain water.
These plants resemble liverworts, but the sporophytes are horn-shaped.
Their photosynthetic cells have one large chloroplast, unlike the many
smaller ones of other plants.
- Early Vascular
- For plants to develop
and grow above a certain height, specialized tissues had to be
developed to absorb water and nutrients from the soil and transport
this material from the ground to the upper areas of the plant. Aquatic
plants used the water, it lived in, to support its structure.
Reproductive material was transported by the water to its destination.
Pollen eliminated the need for the water to transport the gametes,
while the development of the seed allowed the vascular plants to
conquer the land. The diploid sporophyte increased in dominance
relegating the gametophyte to a microscopic structure within the
Vascular Plants: The oldest
fossilized vascular plant Cooksonia was discovered in both
Europe and North American Silurian rock. True roots and leaves were
absent; with the largest species being 50 cm. tall.
These are the most well represented of the seedless plants. Most ferns
have fronds, compound leaves divided into several leaflets. As the
frond develops it uncoils from a structure called a fiddle head. Ferns
are homosporus with the leafy plant the sporophyte. The sporangia are
located on the under surface of the fronds in areas called sori.
The gametophyte is a free-living, small, fragile structure. Water is
necessary for fertilization since the sperm must swim to the
archogonium, where fertilization takes place.
As the climate began to dry out the large Fern forests began to be
replaced by a new type of plant the gymnosperm. This plant had
vascular tissue and a new type of reproductive structure called the
seed. These plants contained these seeds in structures called cones.
The leaves of most of the gymnosperms are needle -like structures.
These needles contained a thick cuticle and reduced stomata. The
conifers are heterosporus (male and female gametes develop from
different types of spores on different structures).
Plants: are the most widespread and
diverse. The division Anthophyta is divided into two classes,
Monocotyledonae and Dicotyledonae. The development of the flower allow
this group of plants take over most biomes on earth. The life cycle of
the Anthophyta is similar to that of the gymnosperms. The gametophyte
stage is small and microscopic. The male gametophyte is the mature
pollen grain while the female gametophyte is the embryo sac, located
in the ovule.