25: Seedless Plants Flashcards
Early Plant Life, Green Algae - Precursors of Land Plants, Bryophytes, Seedless Vascular Plants
What are some examples of seedless plants and where are they found?
Mosses may grow on a tree trunk, and horsetails (Equisetum sp.) may display their jointed stems and spindly leaves across the forest floor, in damp, shaded environments under tree canopies where dryness is rare.
How prevalent are seedless plants?
Today, seedless plants represent only a small fraction of the plants in our environment; yet, 300 million years ago, seedless plants dominated the landscape and grew in the enormous swampy forests of the Carboniferous period. Their decomposition created large deposits of coal that we mine today.
Which challenges did the ancestors of land plants overcome in order to colonize land?
Current evolutionary thought holds that all plants - green algae as well as land dwellers - are monophyletic. 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.
How many species of plants are there?
There are more than 300,000 species of catalogued plants. Of these, more than 260,000 are seed plants.
Why is there disagreement among biologists about whether green algae should be classified as plants?
Most biologists 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. Additionally, green algae lack alternation of generations, sporangia, gametangia, and apical meristem tissues in roots and shoots, as well as waxy cuticles and cell walls with lignin.
Why is there disagreement among biologists whether all algae should be considered plants?
Some scientists consider all algae to be plants, while others assert that only the Charophytes belong in the kingdom Plantae. These divergent opinions are related to the different evolutionary paths to photosynthesis selected for in different types of algae. While all algae are photosynthetic - that is, they contain some form of a chloroplast - they didn’t all become photosynthetic via the same path.
How did green algae evolve?
The ancestors to the green algae became photosynthetic by endosymbiosing a green, photosynthetic bacterium about 1.65 billion years ago. That algal line evolved into the Charophytes, and eventually into the modern mosses, ferns, gymnosperms, and angiosperms. Their evolutionary trajectory was relatively straight and monophyletic.
How did non-green algae evolve?
Other algae - red, brown, golden, stramenopiles, etc. - all became photosynthetic by secondary, or even tertiary, endosymbiotic events; that is, they endosymbiosed cells that had already endosymbiosed a cyanobacterium. 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.
What challenges do land plants face that water plants do not?
Desiccation is a constant danger for an organism exposed to air. Even when parts of a plant are close to a source of water, the aerial structures are likely to dry out. Water also provides buoyancy to organisms. On land, plants need to develop structural support in a medium that does not give the same lift as water. The organism is also subject to bombardment by mutagenic radiation, because air does not filter out ultraviolet rays of sunlight. 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.
What advantages did land offer to plants?
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.
What is an example of tolerance to desiccation used by plants?
Many mosses 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.
What is an example of plants colonizing humid environments to avoid desiccation?
Ferns, which are considered an early lineage of plants, thrive in damp and cool places, such as the understory of temperate forests.
What is an example of resistance to desiccation used by plants?
Cacti, which minimize the loss of water to such an extent that they can survive in extremely dry environments.
Which major adaptations are found in all terrestrial plants?
Alternation of generations, a sporangium in which the spores are formed, a gametangium that produces haploid cells, and apical meristem tissue in roots and shoots. The evolution of a waxy cuticle and a cell wall with lignin also contributed to the success of land plants.
What is an antheridium?
Male gametangium.
What is an archegonium?
Female gametangium.
What is a charophyte?
Other term for green algae; considered the closest relative of land plants.
What does it mean to be diplontic?
Diploid stage is the dominant stage.
What is an embryophyte?
Other name for land plant; embryo is protected and nourished by the sporophyte.
What does it mean to be extant?
Still-living species.
What does it mean to be extinct?
No longer existing species.
What is a gametangium?
Structure on the (multicellular haploid) gametophyte in which gametes are produced by precursor cells via mitosis.
What does it mean to be haplodiplodontic?
Haploid and diploid stages alternate.
What does it mean to be haplontic?
Haploid stage is the dominant stage.
What does it mean to be heterosporous?
Produces two types of spores.
What does it mean to be homosporous?
Produces one type of spore.
What is a megaspore?
Female spore.
What is a microspore?
Male spore.
What is a non-vascular plant?
Plant that lacks vascular tissue, which is formed of specialized cells for the transport of water and nutrients.
What is a seedless vascular plant?
Plant that does not produce seeds.
What is a sporocyte?
Diploid cell that produces spores by meiosis.
What is sporopollenin?
A tough polymer found in the cell walls surrounding the spores of seedless plants.
What is the molecular makeup of sporopollenin?
Sporopollenin is characterized by long chains of organic molecules related to fatty acids and carotenoids; hence the yellow color of most pollen.
What is a vascular plant?
A plant containing a network of cells that conducts water and solutes through the organism.
What are the steps of the alternation of generations between the 1n gametophyte and the 2n sporophyte?
First, there are spores, which undergo mitosis to form a haploid multicellular gametophyte. The gametophyte gives rise to gametes, which undergo syngamy (fusion) with other gametes to form a zygote, which undergoes mitosis to form a diploid multicellular sporophyte. The sporophyte bears the sporangia and undergoes meiosis to form spores.
How noticeable is the gametophyte phase in plants?
It 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 (vascular) plants.
How noticeable is the sporophyte phase in plants?
The sporophyte stage is barely noticeable in lower plants (e.g. mosses, liverworts, and lichens). Towering trees are the diplontic phase in the lifecycles of plants such as sequoias and pines.
How does embryo protection distinguish land plants?
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.
In which major plant group did sporangia first appear?
Land plants.
What does “sporangia” mean?
Literally, “spore in a vessel”, because it is a reproductive sac that contains spores.
What are some examples of polyploid sporophytes?
Durum wheat is tetraploid, bread wheat is hexaploid, and some ferns are 1000-ploid.
How many types of spores are produced in plants?
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. Gametophytes that result from the germination of spores of homosporous plants produce both male and female gametangia, usually on the same individual. In contrast, heterosporous plants produce two morphologically different types of spores.
Which phase is dominant in seedless non-vascular plants?
The gametophyte phase.
In which types of plants is heterospory found?
Heterospory is observed in a few seedless vascular plants and in all seed plants.
In what ways is sporopollenin tough?
Sporopollenin is unusually resistant to chemical and biological degradation. 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.
In which major plant groups is sporopollenin found?
Sporopollenin was once thought to be an innovation of land plants; however, the green algae Coleochaetes forms spores that contain sporopollenin.
How do antheridia interact with archegonia in seedless plants?
The 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.
How do meristems contribute to plant growth?
Shoots and roots of plants increase in length through rapid cell division in a tissue called the apical meristem, which is a small zone of cells found at the shoot tip or root tip. The apical meristem is made of undifferentiated cells that continue to proliferate throughout the life of the plant. Meristematic cells give rise to all the specialized tissues of the organism. Elongation of the shoots and roots allows a plant to access additional space and resources: light in the case of the shoot, and water and minerals in the case of roots. A separate meristem, called the lateral meristem, produces cells that increase the diameter of tree trunks.
What is the function of a root cap?
The root cap protects the fragile apical meristem as the root tip is pushed through the soil by cell elongation.
Why did vascular systems evolve in plants?
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.
Which tissues are found in plant vascular systems?
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.
Why did root systems evolve?
Root systems evolved to take up water and minerals from the soil, and to anchor the increasingly taller shoot in the soil.
What are some of the pros and cons of cuticles?
In land plants, a waxy, waterproof cover called a cuticle 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 appeared in plants as they moved away from moist environments to drier habitats.
How do land plants protect themselves from UVB light?
Water filters ultraviolet-B (UVB) light, which is harmful 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: pigments that absorb UV wavelengths of light and protect the aerial parts of plants from photodynamic damage.
How do plants deter predation?
Plants cannot avoid being eaten by animals. Instead, they synthesize a large range of poisonous secondary metabolites: complex organic molecules such as alkaloids, whose noxious smells and unpleasant taste deter animals. These toxic compounds can also cause severe diseases and even death, thus discouraging predation. Humans have used many of these compounds for centuries as drugs, medications, or spices.
How was plant evolution influenced by animal evolution?
As plants co-evolved with animals, the development of sweet and nutritious metabolites lured animals into providing valuable assistance in dispersing pollen grains, fruit, or seeds.
What are the periods of the Paleozoic era?
It starts with the Cambrian period, followed by the Ordovician, Silurian, Devonian, Carboniferous, and Permian.
When did land plants evolve?
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.
What is one of the most important sources of information regarding the evolution of land plants?
One of the richest sources of information is the Rhynie chert, a sedimentary rock deposit found in Rhynie, Scotland, where embedded fossils of some of the earliest vascular plants have been identified.
What are the extinct vascular plants?
The extinct vascular plants, classified as zosterophylls and trimerophytes, most probably lacked true leaves and roots and formed low vegetation mats similar in size to modern-day mosses, although some trimerophytes could reach one meter in height.
What is Cooksonia?
The later genus Cooksonia (beyond zosterophylls and trimerophytes), which flourished during the Silurian, has been extensively studied from well-preserved samples. Imprints of Cooksonia show slender branching stems ending in what appear to be sporangia. From the recovered specimens, it is not possible to establish for certain whether Cooksonia possessed vascular tissues.
Which land plants were found by the end of the Devonian period?
Fossils indicate that by the end of the Devonian period, ferns, horsetails, and seed plants populated the landscape, giving rise to trees and forests. This luxuriant vegetation helped enrich the atmosphere in oxygen, making it easier for air-breathing animals to colonize dry land. Plants also established early symbiotic relationships with fungi, creating mycorrhizae.
What is paleobotany?
The study of extinct plants.
What are the goals of paleobotany?
Paleobotany addresses the questions of how organisms acquired traits that allow them to colonize new environments - and how the contemporary ecosystem is shaped - through the analysis of fossilized specimens retrieved from field studies, reconstituting the morphology of organisms that disappeared long ago. Paleobotanists trace the evolution of plants by following the modifications in plant morphology: shedding light on the connection between existing plants by identifying common ancestors that display the same traits. This field seeks to find transitional species that bridge gaps in the path to the development of modern organisms
How are fossils formed?
Fossils are formed when organisms are trapped in sediments or environments where their shapes are preserved.
In what ways do paleobotanists work with fossils?
Paleobotanists collect fossil specimens in the field and place them in the context of the geological sediments and other fossilized organisms surrounding them. The activity requires great care to preserve the integrity of the delicate fossils and the layers of rock in which they are found.
What is required for the fossilization of molecular structures?
Preservation of molecular structures requires an environment free of oxygen, since oxidation and degradation of material through the activity of microorganisms depend on its presence.
What is an example of the use of analytical chemistry and molecular biology in paleobotany?
The identification of oleanane, a compound that deters pests. Up to this point, oleanane appeared to be unique to flowering plants; however, it has now been recovered from sediments dating from the Permian, much earlier than the current dates given for the appearance of the first flowering plants.
In what ways do paleobotanists work with DNA?
Paleobotanists can study fossil DNA, which can yield a large amount of information, by analyzing and comparing the DNA sequences of extinct plants with those of living and related organisms. Through this analysis, evolutionary relationships can be built for plant lineages.
Why are some paleobotanists skeptical of the conclusions drawn from the analysis of molecular fossils?
The chemical materials of interest degrade rapidly when exposed to air during their initial isolation, as well as in further manipulations. There is always a high risk of contaminating the specimens with extraneous material, mostly from microorganisms.
What are streptophytes?
The green algae and land plants are grouped together into a subphylum called the Streptophytina, and thus are called streptophytes.
What are the major groups in land plant classification?
Land plants are classified into two major groups according to the absence or presence of vascular tissue (vascular and non-vascular plants).
What are some examples of seedless, non-vascular plants?
Liverworts, mosses, and hornworts are seedless, non-vascular plants that likely appeared early in land plant evolution.
What are some examples of seedless vascular plants?
Lycophytes and pterophytes.
