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An advanced placement biology on vertebrate structural adaptation for terrestrial life

Identify and analyze specific plant adaptations for life on land Identify dominant plant taxa during geological time periods Differentiate the major plant taxa based on evolutionary adaptations and life cycles Compare and contrast angiosperms and gymnosperms Seedless plants, like these horsetails Equisetum sp.

There are more than 300,000 species of catalogued plants. Of these, more than 260,000 are seed plants. Mosses, ferns, conifers, and flowering plants are all members of the plant kingdom. Most biologists also consider green algae to be plants, although others exclude all algae from the plant kingdom. The reason for this disagreement stems from the fact that only green algae, the Charophytes, share common characteristics with land plants such as using chlorophyll a and b plus carotene in the same proportion as plants.

These characteristics are absent in other types of algae. Current evolutionary thought holds that all plants, green algae as well as land dwellers, are monophyletic; that is, they are descendants of a single common ancestor.

The evolutionary transition from water to land imposed severe constraints on plants. They had to develop strategies to avoid drying out, to disperse reproductive cells in air, for structural support, and for capturing and filtering sunlight.

While seed plants developed adaptations that allowed them to populate even the most arid habitats on Earth, full independence from water did not happen in all plants. Most seedless plants still require a moist environment. Algae and Evolutionary Paths to Photosynthesis The ancestors of green algae became photosynthetic by endosymbiosing a green, photosynthetic bacterium about 1.

  • The gametophyte phase is dominant in these plants;
  • Because variation in weather conditions might affect oviposition choices, we always tested frogs from all three ponds on each night;
  • Alternation of Generations Alternation of generations describes a life cycle in which an organism has both haploid and diploid multicellular stages.

That algal line evolved into the Charophytes, and eventually into the modern mosses, ferns, gymnosperms, and angiosperms. In contrast, the other algae: These latecomers to photosynthesis are parallels to the Charophytes in terms of autotrophy, but they did not expand to the same extent as the Charophytes, nor did they colonize the land.

Go to this interactive website to get a more in-depth view of the Charophytes. Plant Adaptations to Life on Land As organisms adapted to life on land, they had to contend with several challenges in the terrestrial environment. Desiccation, or drying out, is a constant danger for an organism exposed to air.

  1. Liverworts, mosses, and hornworts are seedless, non-vascular plants that likely appeared early in land plant evolution. Like flowers, fruit can vary tremendously in appearance, size, smell, and taste.
  2. By 2080—2099, the air temperature during June, July, and August, peak breeding months for D. In a further division, land plants are classified into two major groups according to the absence or presence of vascular tissue.
  3. The male gametangium antheridium releases sperm.
  4. After germinating from a spore, the resulting gametophyte produces both male and female gametangia, usually on the same individual. Less predictable rainfall and more disturbed habitats will increase the mortality of terrestrially laid eggs 45 , but reproductive mode variation may help D.
  5. Green, flat structures— resembling true leaves, but lacking vascular tissue— are attached in a spiral to a central stalk.

Even when parts of a plant are close to a source of water, the aerial structures are likely to dry out. In land plants, a waxy, waterproof cover called a cuticle evolved which protects the leaves and stems from desiccation.

However, the cuticle also prevents intake of carbon dioxide needed for the synthesis of carbohydrates through photosynthesis. To overcome this, stomata or pores that open and close to regulate traffic of gases and water vapor. Water also provides buoyancy to organisms.

On land, plants developed structural support in a medium that does not give the same lift as water: Additionally, the male gametes must reach the female gametes using new strategies, because swimming is no longer possible.

Therefore, both gametes and zygotes must be protected from desiccation. The successful land plants developed strategies to deal with all of these challenges. Water filters ultraviolet-B UVB light, which is harmful mutagenic to all organisms, especially those that must absorb light to survive.

This filtering does not occur for land plants. This presented an additional challenge to land colonization, which was met by the evolution of biosynthetic pathways for the synthesis of protective flavonoids and other compounds: In small plants such as single-celled algae, simple diffusion suffices to distribute water and nutrients throughout the organism. However, for plants to evolve larger forms, the evolution of vascular tissue for the distribution of water and solutes was a prerequisite.

The vascular system contains xylem and phloem tissues. Xylem conducts water and minerals absorbed from the soil up to the shoot, while phloem transports food derived from photosynthesis throughout the entire plant.

A root system evolved to take up water and minerals from the soil, and to anchor the increasingly taller shoot in the soil. Not all adaptations appeared at once. Some species never moved very far from the aquatic environment, whereas others went on to conquer the driest environments on Earth. To balance these survival challenges, life on land offers several advantages.

First, sunlight is abundant. Water acts as a filter, altering the spectral quality of light absorbed by the photosynthetic pigment chlorophyll.

Second, carbon dioxide is more readily available in air than in water, since it diffuses faster in air. Third, land plants evolved before land animals; therefore, until dry land was colonized by animals, no predators threatened plant life.

This situation changed as animals emerged from the water and fed on the abundant sources of nutrients in the established flora.

Reproductive mode plasticity: Aquatic and terrestrial oviposition in a treefrog

In turn, plants developed strategies to deter predation: Early land plants, like the early land animals, did not live very far from an abundant source of water and developed survival strategies to combat dryness. One of these strategies is called tolerance. Many mosses, for example, can dry out to a brown and brittle mat, but as soon as rain or a flood makes water available, mosses will absorb it and are restored to their healthy green appearance.

Another strategy is to colonize environments with high humidity, where droughts are uncommon. Ferns, which are considered an early lineage of plants, thrive in damp and cool places such as the understory of temperate forests.

Alternation of Generations

Later, plants moved away from moist or aquatic environments using resistance to desiccation, rather than tolerance.

These plants, like cacti, minimize the loss of water to such an extent they can survive in extremely dry environments. The most successful adaptation solution was the development of new structures that gave plants the advantage when colonizing new and dry environments. Four major adaptations are found in all terrestrial plants: The evolution of a waxy cuticle and a cell wall with lignin also contributed to the success of land plants. These adaptations are noticeably lacking in the closely related green algae- another reason for the debate over their placement in the plant kingdom.

Alternation of Generations Alternation of generations describes a life cycle in which an organism has both haploid and diploid multicellular stages. Alternation of generations between the 1n gametophyte and 2n sporophyte is shown. Peter Coxhead Haplontic refers to a lifecycle in which there is a dominant haploid stage, and diplontic refers to a lifecycle in which the diploid is the dominant life stage. Most plants exhibit alternation of generations, which is described as haplodiplodontic: The gametophyte gives rise to the gametes reproductive cells by mitosis.

This can be the most obvious phase of the life cycle of the plant, as in the mosses, or it can occur in a microscopic structure, such as a pollen grain, in the higher plants a common collective term for the vascular plants. The sporophyte stage is barely noticeable in lower plants the collective term for the plant groups of mosses, liverworts, and lichens.

Towering trees are the diplontic phase in the lifecycles of plants such as sequoias and pines. Protection of the embryo is a major requirement for land plants. The vulnerable embryo must be sheltered from desiccation and other environmental hazards.

In both seedless and seed plants, the female gametophyte provides protection and nutrients to the embryo as it develops into the new generation of sporophyte. This distinguishing feature of land plants gave the group its alternate name of embryophytes.

Sporangia in Seedless Plants The sporophyte of seedless plants is diploid and results from syngamy fusion of two gametes. The sporophyte bears the sporangia singular, sporangium: Inside the multicellular sporangia, the diploid sporocytes, or mother cells, produce haploid spores by meiosis, where the 2n chromosome number is reduced to 1n.

The spores are later released by the sporangia and disperse in the environment. Two different types of spores are produced in land plants, resulting in the separation of sexes at different points in the lifecycle. Seedless non-vascular plants produce only one kind of spore and are called homosporous. The gametophyte phase is dominant in these plants. After germinating from a spore, the resulting gametophyte produces both male and female gametangia, usually on the same individual.

In contrast, heterosporous plants produce two morphologically different types of spores.

The male spores are called microspores, because of their smaller size, and develop into the male gametophyte; the comparatively larger megaspores develop into the female gametophyte Note: Heterospory is observed in a few seedless vascular plants and in all seed plants.

Spore-producing sacs called sporangia grow at the ends of long, thin stalks in this photo of the moss Esporangios bryum. Javier Martin When the haploid spore germinates in a hospitable environment, it generates a multicellular gametophyte by mitosis. The gametophyte supports the zygote formed from the fusion of gametes and the resulting young sporophyte vegetative form.

The cycle then begins anew. The spores of seedless plants are surrounded by thick cell walls containing a tough polymer known as sporopollenin. This complex substance is characterized by long chains of organic molecules related to fatty acids and carotenoids: Sporopollenin is unusually resistant to chemical and biological degradation.

Terrestrial reproduction as an adaptation to steep terrain in African toads

In seed plants, which use pollen to transfer the male sperm to the female egg, the toughness of sporopollenin explains the existence of well-preserved pollen fossils. Sporopollenin was once thought to be an innovation of land plants; however, the green algae Coleochaetes forms spores that contain sporopollenin.

Gametangia in Seedless Plants Gametangia singular, gametangium are structures observed on multicellular haploid gametophytes. In the gametangia, precursor cells give rise to gametes by mitosis. The male gametangium antheridium releases sperm. Many seedless plants produce sperm equipped with flagella that enable them to swim in a moist environment to the archegonia: The embryo develops inside the archegonium as the sporophyte.

Gametangia are prominent in seedless plants, but are very rarely found in seed plants.

1. Introduction

Evolution of Land Plants No discussion of the evolution of plants on land can be undertaken without a brief review of the timeline of the geological eras.

The early era, known as the Paleozoic, is divided into six periods. The major event to mark the Ordovician, more than 500 million years ago, was the colonization of land by the ancestors of modern land plants.

Fossilized cells, cuticles, and spores of early land plants have been dated as far back as the Ordovician period in the early Paleozoic era.

The oldest-known vascular plants have been identified in deposits from the Devonian.