Chapter 37 evolution of plants

The Plant Kingdom

· · A plant is a photosynthetic eukaryote that uses chlorophylls a and b, stores carbohydrates, and develops from an embryo protected by tissues of the parent plant.

· · The kingdom of Plantae is monophyletic, forming a single branch of the evolutionary tree.

· · Because of their development from embryos, plants are sometimes referred to as embryophytes.

There are twelve surviving phyla of plants

· · The surviving members of the kingdom Plantae fall naturally into 12 phyla.

· · The nine plant phyla whose members possess well-developed vascular systems that transport materials throughout the plant body are called the tracheophytes.

· · The remaining three phyla (liverworts, hornworts, and mosses) lack tracheids and are collectively referred to as the nontracheophytes.

Life cycles of plants feature alternation of generations

· · The alternation of generations is a universal feature of the life cycles of plants

· · The multicellular, diploid plant is called the sporophyte.

· Cells contained in the sporangia on the sporophyte produce haploid, unicellular spores through meiosis.

· · The multicellular, haploid plant formed by mitosis and cytokinesis of a spore is called the gametophyte. The gametophyte produces haploid gametes.

· · The fusion of two gametes results in the formation of a diploid cell, the zygote, and the cycle repeats.

· · The sporophyte generation extends from the zygote through the adult, multicellular, diploid plant; the gametophyte generation extends from the spore through the adult, multicellular, haploid plant to the gamete.

The Plantae arose from a green algal lineage

· · Evidence indicates that the closest living relatives of the plants are a group of green algae called charophytes.

· · The ancestral green algae lived at the margins of ponds or marshes. From these marginal habitats, early plants made the move onto land.

The Conquest of the Land

· · Plants or their immediate ancestors pioneered and modified the terrestrial environment.

· · The availability of water that was essential for life was a key difference between the aquatic and terrestrial environments.

· · Whereas water defines the aquatic environment, it can be difficult to find and retain in the terrestrial environment.

Adaptations to life on land distinguish plants from green algae

· · Most of the characteristics that distinguish plants from the green algae are evolutionary adaptations to life on land.

· · Many of the following characteristics proved adaptive to land plants.

· · The cuticle, a waxy covering that prevents drying

· · Gametangia, cases that enclose plant gametes and prevent drying

· · Embryos, young sporophytes contained within a protective structure

· · Pigments that afford protection against the mutagenic ultraviolet radiation that bathes the terrestrial environment

· · Thick spore walls to prevent drying and resist decay

Most present-day plants have vascular tissue

· · The first plants lacked both water-conducting and food-conducting cells.

· · The nontracheophytes utilize the following behaviors and structures to obtain water and minerals in the absence of a vascular system:

· · Many nontracheophytes grow in dense masses through which water can move by capillary action.

· · They have leaflike structures that catch and hold water that splashes onto them.

· · They are small enough that minerals can be distributed evenly by diffusion.

· · The tracheophytes differ from the nontracheophytes in many ways, one of which is the possession of a well-developed vascular system.

· · This vascular system consists of two specialized tissues used in the transport of materials from one part of the plant to another.

· · The phloem is used to conduct products of photosynthesis from sites where they are produced or released to sites where they are used or stored.

· · The xylem conducts water and minerals from the soil to the aerial parts of the plants.

· · The xylem also provides support in the terrestrial environment.

· · The nontracheophyte plants evolved tens of millions of years before the tracheophyte plants.

The Nontracheophytes: Liverworts, Hornworts, and Mosses

· · The nontracheophytes usually grow in dense mats in moist habitats.

· · The nontracheophytes are generally small, most likely because they lack an efficient system for conducting water and minerals from the soil to distant parts of the plant body.

· · Layers of maternal tissue prevent loss of water from the embryo.

· · The nontracheophytes have a thin cuticle that is not highly effective in retarding water loss.

· · The nontracheophytes are widespread across six continents and exist locally on the coast of Antarctica.

Nontracheophyte sporophytes are dependent on gametophytes

· · In nontracheophytes(non-vascular), the familiar green structure visible to the naked eye is the gametophyte. ·

· Gametangia are specialized sex organs where gametes are formed.

· · The archegonium is a multicellular, flask-shaped female sex organ with a long neck and a swollen base that contains a single egg.

· · The antheridium is a male sex organ in which sperm are produced in large numbers.

· · The sporophyte produces a sporangium, or capsule, within which meiotic divisions produce spores and thus the next gametophyte generation.

Liverworts are the most ancient surviving plant lineage

· · The common name for members of the phylum Hepatophyta is liverworts.

· · Rhizoids are water-absorbing filaments that are found on the lower surfaces of the simplest liverwort gametophytes.

· · Liverwort sporophytes have a stalk that connects the capsule and the foot. This capsule can elongate to raise the capsule above ground level, aiding in the dispersion of spores when they are released.

· · Other liverworts utilize structures called elaters, located within the capsule, to disseminate their spores.

· · Elaters are long cells that, in dry conditions, shrink and cause a springlike compression of a helical structure present in their cell walls.

· · Upon sufficient stress, this compressed structure snaps back into its original position, throwing spores in all directions.

Hornworts evolved stomata as an adaptation to terrestrial life

· · The hornworts are the common name for the phylum Anthocerophyta.

· · The hornworts, along with the mosses and the tracheophytes, all have an adaptation to life on land not found in the liverworts.

· · These groups all posses stomata that allow the uptake of CO₂ and the release of O₂, but that can close to prevent excessive water loss.

· · There are two characteristics that distinguish hornworts from liverworts and mosses.

· · The cells of hornworts contain a single large, platelike chloroplast, whereas the other nontracheophytes contain numerous small, lens-shaped chloroplasts.

· · Of all of the nontracheophyte sporophytes, the hornworts come closest to being capable of infinite growth.

· · Cyanobacteria often populate internal, mucilage-filled cavities within hornworts. These cyanobacteria are able to fix atmospheric nitrogen gas into a nutrient form that can be used by the hornwort.

· · The exact evolutionary status of hornworts is still unresolved.

Water- and sugar-transport mechanisms emerged in the mosses

· · Members of the phylum Bryophyta are commonly known as mosses.

· · The mosses are sister to the tracheophytes.

· · Hydroid cells, a type of cell found in many mosses, are a likely progenitor of the characteristic water-conducting cell of the tracheophytes.

· · When hydroid cells die, they leave a tiny channel through which water can flow.

· · The sporophytes of the mosses and tracheophytes grow by apical cell division, where a region at the growing tip provides an organized pattern of cell division, elongation, and differentiation.

· · The moss gametophyte that develops following spore germination is a branched, filamentous structure called a protonema.

· · Some filaments in the protonema are photosynthetic, while others, called rhizoids, are nonphotosynthetic and anchor the protonema to the substrate.

· · The tips of the photosynthetic filament eventually form buds.

· · The buds differentiate into a distinct tip and produce a leafy moss shoot with leaflike structures arranged spirally.

· · These leafy shoots produce the antheridia and the archegonia.

· · Moss sporophyte stalks grow at their apical end, as do tracheophyte sporophytes.

· · Sporophyte development in most mosses results in the formation of an absorptive foot, a stalk, and a swollen capsule at the tip.

· · After meiosis and spore development are complete, the top of the capsule is shed.

· · A series of toothlike structures surrounds the opening of the capsule and digs into the mass of spores when the atmosphere is dry.

· · When the atmosphere becomes moist, they fling out, thus dispersing the spores when conditions favor germination.

· · The mosses have simple systems of internal transport, but because they lack xylem and phloem, they are not tracheophytes.

Introducing the Tracheophytes

· · Although the tracheophytes are a large and diverse group, their appearance can be attributed to a single evolutionary event that occurred sometime during the Paleozoic era.

· · The sporophyte generation of a now-extinct organism produced a new cell type, called the tracheid.

· · The tracheid is the principle water-conducting element in the xylem in all tracheophytes except the angiosperms.

· · The appearance of tissue comprised of tracheids had two important consequences:

· · It provided a pathway for long-distance transport of water and minerals from a source of supply to a source of need.

· · It also provided rigid structural support, something almost completely unnecessary in the aquatic green algae.

· · The tracheid set the stage for the complete and permanent invasion of land by plants.

· · The tracheophytes also feature a branching, independent sporophyte.

· · There are nine distinct phyla that are present-day evolutionary descendants of the early tracheophytes. (See Figure 28.8.)

· · These nine phyla can be sorted into two groups: those that produce seeds and those that do not.

· · In the nonseed tracheophytes, the haploid and diploid generations are independent at maturity.

· · The sporophyte of the nonseed tracheophytes is the large and obvious plant, while the gametophytes are rarely more than 1 or 2 centimeters long and are short-lived.

· · The nonseed tracheophytes must have an aqueous environment for at least one stage of their life cycle because a motile, flagellated sperm accomplishes fertilization.

Tracheophytes have been evolving for almost half a billion years

· · The plant kingdom successfully invaded the terrestrial environment between 400 and 500 million years ago.

· · During the Devonian period, 409 to 354 million years ago, some remarkable developments arose.

· · The appearance and proliferation of the club mosses (lycopods), horsetails, and ferns made the environment more hospitable to animals.

· · Trees of various kinds appeared during this period.

The earliest tracheophytes lacked roots and leaves

· · The first tracheophytes belonged to the now-extinct phylum Rhyniophyta.

· · They had early versions of the structural features found in all other tracheophyte phyla.

· · In 1917, the British paleobotanists Kidston and Lang found well-preserved tracheophyte fossils embedded in Devonian rocks near Rhynie, Scotland.

· · The fossil plants had a simple vascular system of xylem and phloem.

· · The plants lacked leaves and roots and were apparently anchored to the soil by horizontal portions of stem called rhizomes.

· · The presence of xylem indicated that these plants (named Rhynia) were tracheophytes.

· · It was initially unknown whether the plants were sporophytes or gametophytes.

· · Inspection of fossil sporangia showed that the spores were in groups of four.

· · Most living nonseed tracheophytes show an arrangement of spores such as this only in the sporophyte.

· · Because a group of four closely packed spores is found only immediately after meiosis, and because the only plant that produces such a group of four must be a diploid sporophyte, it was concluded that the Rhynie fossils must be sporophytes.

Early tracheophytes added new features

· · Three new phyla of tracheophytes appeared during the Devonian period: the Lycophyta, the Sphenophyta, and the Pterophyta.

· · These new groups had true roots, true leaves, and a differentiation between two types of spores.

· · The origin of roots:

· · the ancestors of the first tracheophytes had grown by branching dichotomously.

· · If a branch were to bend, penetrate the soil, and branch there, it could serve to anchor the plant firmly and possibly absorb water and minerals.

· · The underground and aboveground branches would be subjected to sharply different environments and thus very different selection during the succeeding millions of years.

· · Thus, the shoot and root systems diverged in structure and evolved distinct internal and external anatomies.

· · The origin of true leaves:

· · A leaf is a flattened photosynthetic structure emerging laterally from a main axis or stem and possessing true vascular tissue.

· Homospory and heterospory:

· · Plants that bear a single type of spore are said to be homosporous.

· · Plants that bear two distinct types of spores evolved later, and are said to be heterosporous.

· · In heterosporous plants, the megaspore develops into a larger, specifically female gametophyte, while the microspore develops into the smaller, male gametophyte.

· · The most ancient tracheophytes were all homosporous.

· · Heterospory evolved independently and repeatedly, suggesting that it affords selective advantages.

The Surviving Nonseed Tracheophytes

· · Today, the ferns are the most abundant and diverse phylum of the nonseed tracheophytes, but club mosses and horsetails used to be the dominant elements of Earth’s vegetation.

The club mosses are sister to the other tracheophytes

· · The club mosses (phylum Lycophyta) diverged earlier than all other living tracheophytes.

· · They bear simple leaves, exhibit apical growth, and have roots that branch dichotomously.

· · Sporangia in club mosses are contained within conelike structures called strobili.

· · The fossilized spores of a tree lycopod called Lepidodendron form an abundant type of coal.

Horsetails grow at the base of their segments

· · The horsetails (phylum Sphenophyta) have true roots that branch irregularly, bear simple leaves that form circles around the stem, and exhibit basal growth.

Ferns evolved large, complex leaves

· · The sporophytes of the ferns (phylum Pterophyta) typically have large leaves with branching vascular strands.

· · Ferns are characterized by fronds and male gametes that require water for transportation to the female gametes.

· · Sporangia are found on the undersurfaces of leaves; in most species, they are clustered in groups called sori.

The sporophyte generation dominates the fern life cycle

· · Most ferns are homosporous, but there are two groups of aquatic ferns that are heterosporous: the Marsileales and Salviniales.