The Early Plants

Classification of plants

Division
Examples
# of Species
Bryophyte
Mosses
10,000
Hepatophyta
Liverworts
6,500
Anthocerophyta
Hornworts
100
Psilophyta
Whiskferns
10-13
Lycophyta
Club Mosses
1,000
Sphenophyta
Horsetails
15
Pterophyta
Ferns
12,000
Coniferophyta
Conifers
550
Cycadophyta
Cycads
100
Ginkophyta
Ginkgo
One
Gentophyta
Gentae
70
Anthophyta
Flowering Plants
235,000

purple : nonvascular plants,

brown: vascular seed-less plants,

green: vascular seed plants

Plant History:

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:

Bryophytes: 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.

Mosses. 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.

Liverworts. 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.

Hornworts. 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 Plants:

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 sporophyte.

Earliest 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.

Ferns. 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.

Gymnosperms: 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).

Flowering 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.