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BIOL 2306 EL

Adaptations That Occurred In Plants from Their Aquatic Origin to the Flowering Plants & How They Affected Their Diversity Plant Diversity Megan Vaz

In this paper the adaptations and evolution of aquatic plants are discussed based on features that they have developed to survive in the new land.

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Megan Vaz Dr. Nasserula, S. BIOL 2306 EL December 16th, 2020 Adaptations That Occurred In Plants from Their Aquatic Origin to the Flowering Plants & How They Affected Their Diversity In the terrestrial ecosystem, plants are the most dominant component. Although modern day flowering plants differ from aquatic plants; they are in fact very similar. This is because terrestrial flowering plants descended from plants that once inhibited aquatic environments. Over the years, terrestrial plants have adapted and evolved into what is considered the main source of energy for the majority of other terrestrial organisms. The primary focus of this essay will be to examine the flowering plants evolutionary history, and their adaptations from their aquatic ancestors due to selective forces that have shaped their certain aspects. These changes have allowed these flowering plants to diversify because of previously existed plants in the plantae kingdom. However, the evolutionary transition from water to land imposed severe constraints on plants. Flowering plants 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. An example of an adaptation in plants from an aquatic to terrestrial stage can be eminent through algae; which is a specific member of the plantae kingdom. In this essay we will reflect on the evolutionary characteristics towards some of these plants; such as morphology, reproduction and life cycle. Along with the benefits and challenges of living in a terrestrial environment and the generations which have undergone some changes in dominancy of plants evolve.

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Figure 1. Shows the beginning and the branches of how plants dispersed and evolved Plants have been evolving over the last 450 million years. According to research, the first vascular plants may have appeared in the late Ordovician period of the Paleozoic Era. The kingdom Plantae constitutes a very large and diverse group of organisms with more than 300,000 species of seedless plants (nonvascular), and with more than 260,000 seeded plants (vascular). Three hundred million years ago the seedless plants dominated the landscape and grew in the enormous swampy forests of the Carboniferous period. Eventually, most modern day plants which are considered seed plants, which have descended from ancestral seedless plants that once, lived in an aquatic environment. Today, the non-vascular plants represent a small fraction of the plants which exist in our environment, whereas the vascular plants make up 90 percent of the earth’s vegetation. Nonvascular plants are classified into four main categories: algae (especially the green algae), Byrophytes (mosses), Anthocerophyta (hornworts), and Marchantiophyta (liverworts). It was suggested that the aquatic ancestors (green algae) ranged from a little moss

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relative called liverwort, to a tall grassy plant known as horsetails, to some sort of gymnosperm (pines or firs) and then evolved into a proper flowering plant (the angiosperm).

Figure 2. Major divisions of land plants: Land plants are categorized by presence or absence of vascular tissue and their reproduction with or without the use of seeds.

Researchers believed that plants that were once confined to the water; evolved from green algae that looked like the Chara (as seen in the picture below).

Figure 3. Chara The freshwater green algae, which is considered a protist (diverse collection of organisms), aapproximately 450-500 million years ago, ancestral charophytes emerged onto land and

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ultimately gave rise to the first true plants, thus making them the closest relatives of the plant kingdom. There significant evidence (e.g., chemical, structural, and genetic data) to conclude the close evolutionary relationship between charophytes and plants. They concluded that both green algae and all plants are monophyletic; which means they are descendents of a single common ancestor. One large piece of evidence that suggests they are similar is that they both have cellulose in their cell walls, and share many of the same chemicals that give them colour. Chlorophytes (green algae) and today’s plants have the same photosynthetic pigments; known as chlorophyll b. Although chlorophyll a is common; chlorophyll b is only shared by green algae and terrestrial plants. Another characteristic that these two species share is the carbon storage in the form of starch and formation of the cell plate at cytokinesis. Also researchers have determined that after analyzing the RNA and DNA sequences of both green algae and terrestrial plants it was clear to see how similar they were. However there are significant differences which resulted in the evolutionary difference of aquatic plants versus flowering plants. Seeded vascular plants or commonly known as flowering plants, are this centuries dominator. Flowering plants include anything from gardenias to grasses; and water lilies to wheat and all belong in a large and diverse group called angiosperms (Greek for ‘vessel’ & ‘seeds’). An angiosperm is a plant that has flowers and produces seeds enclosed within a carpel. Around 140 million years ago during the late Cretaceous period, the first flowering plant (basal angiosperm) burst into life on Earth. Note: The flowering plants would only exist for about 90 seconds.

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Figure 4. Which plants species were seen in which era

Botanists have determined that the enormous magnolia was thought to be the poster bloom for the earliest flowering plants. It was depicted on many dinosaur infested murals.

Figure 5. Enormous Magnolia

Figure 6. Magnolias Seen in Dinosaur Painting

As time went on, many other hypothesis surfaced by paleobotanists (scientists who study ancient plants) that concluded the first angiosperm was actually an odorless, nectar-less, petalless, aqueous and pollinator-less flower. In short they believe it was an unrecognizable flower and this lead them to the evidence of a leaf imprint that showed a Ficus speciosissima, an

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angiosperm that flourished during the Cretaceous period (A large number of pollinating insects also appeared during this same time).

Figure 6. Fossil Evidence of angiosperms: This leaf imprint shows a Ficus speciosissima, an angiosperm that flourished during the Cretaceous period. A large number of pollinating insects also appeared during this same time.

This evolution of flowering plants occurred by a stepwise development of physical structures and reproductive mechanisms. The embryo protection developed prior to the development of vascular plants. These reproductive mechanisms consist of vascular tissue, seed production, and flowering. Vascular plants are any plants that posse’s vascular tissue (xylem and phloem). Although gymnosperms and angiosperms are both vascular seed plants, they differ very much. The main difference between the two is how their seeds develop. Gymnosperm seed are usually formed in unisexual cones which are commonly known as strobili and they lack fruits and flower. Whereas the angiosperms form a sister clade and are developed in the ovaries of flowers and are surrounded by a protective fruit.

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Figure 7. "Ovule-Gymno-Angio-en" by derivative work: Qef (talk) Ovule-GymnoAngio-fr.svg: Original uploader was Tameeria at en.wikipedia. Translation and vectorisation by Cehagenmerak. - Ovule-Gymno-Angio-fr.svg. Licensed under Public Domain via Commons. So if the first flowering plant didn’t really exhibit any flowering appearances to the human eye, what exactly made them a flower? The answer is the protective fruit. As previously mentioned the definition of an angiosperm is a plant that has flowers and produces seeds enclosed within a carpel, which usually contains a stigma and style ( the sticky bit that protrudes from the flower to receive pollen) . Insects pollinate the flowers by deposition pollen on the

stigma. Once the pollen grains germinate and send out germ tubes that travel down the style to the ovary. Once the eggs are fertilized the whole structure becomes seed-containing fruit; a fruit is simply a fertilized and ripened ovary. In the beginning plants just like animals could not live far from a water source. These flowering plant species were known to be the early evolved but still aquatic monocot species. They included water lilies, grasses, magnolias, and one extremely important one the Amborella (which today is known as the woody shrub-found on the Pacific French archipelago island of

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New Caldonia). The Amborella genome indicates that the first angiosperm appeared when the ancestral gymnosperm underwent a ‘whole genome doubling’ process approximately 200 years ago.

Figure 8. Phylogentic genome analysis of plant evolution due to the division of gymnosperms and angiosperms based on the first founding flower Amborella

Over time they gradually pushed on to land. They adapted to life on land by developing many fundamental structures and strategies to avoid drying out, disperse reproductive cells into the air in order to reproduce, structural support, and tools for capturing and filtering enough sunlight. . Such as:

“water repellent cuticle, stomata to regulate water evaporation, specialized cells to provide rigid support against gravity, specialized structures to collect sunlight, alternation of haploid and diploid generations, sexual organs, a multicellular embryo protected by parental tissue, a vascular transport system, and rhizoids, roots and root hairs essential for the absorption of nutrients and anchoring to the substrate” (Mario A. Arteaga-Vazquez). . These changes separate them from their ancestor; the green algae (their relative). There are four main adaptations the plants underwent to become a eudicot flower were:

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1. The embryo developed inside the female part of the plant after fertilization. Algae does not keep the embryo inside of itself, but instead releases it into the water. This was the first feature to evolve and separated plants from algae. (This adaption is shared by all plants). 2. Plants had to evolve from living in water to living on land and therefore the waxy layer known as a cuticle evolved to help seal in water in the plants and prevent water loss. The cuticle also prevents gasses from entering and leaving the plants easily. The cuticle is translucent and therefore does not block the sun’s rays from reaching the photosynthetic cells. It protects the mesophyll layer which contains the cells. 3. To allow plants to retain water and exchange gases; small pores (holes) in the leaves called the stomata evolved as well (Figure below).

Figure 9. Stomata

The stomata of the plants are able to open and close depending on the weather. When it is hot and dry, the stomata stays closed to contain the water inside of the plant. When the weather is cool, the stomata opens again to let carbon dioxide in and oxygen out. 4. A later adaption for life on lad was the evolution of the vascular tissue which was previously mentioned. The vascular tissue is a specialized tissue that transports water, nutrients, and food in plants. In algae the vascular tissue is not needed since the body of the algae is constantly in contact with water. However in land plants the water is only found deep in the ground. Vascular tissues take water and nutrients from the ground up into the plant, while transporting food down from the leaves into the rest of the plant. The

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two vascular tissues are called: xylem and phloem. Xylem is the one responsible for the transport of water and nutrients from the roots to the rest of the plant. The Phloem carries the sugars made in the leaves to plant parts that are in need of them. In conclusion the “first flower” had not been perched on a tree but rather hidden inside a humble aquatic weed. The adaptations that flowering plants underwent (embryo retention, a cuticle, stomata and vascular tissue) where extremely vital in its survival thus leading to the profound changes of terrestrial ecosystems. The points that have been demonstrated, it is clear that plants arose from their ancestor the green algae. Therefore flowering plants have become the dominating species on land and affects Earth’s diversity not by just aesthetic beauty but also as and the main source of food for animals.

Figure 10: https://www.google.com/url?sa=i&url=https%3A%2F %2Fen.wikipedia.org%2Fwiki %2FPlant&psig=AOvVaw2qbRUDNOVaE8ByD4XbfZD5&ust=1603316865133 000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCPiXgPySxOwCFQAAA AAdAAAAABAX

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References https://www.frontiersin.org/research-topics/4150/charophytes-evolutionary-ancestors-of-plantsand-emerging-models-for-plant-research https://courses.lumenlearning.com/boundless-biology/chapter/early-plant-life/ https://courses.lumenlearning.com/boundless-biology/chapter/evolution-of-seed-plants/ https://wikispaces.psu.edu/display/BIOL110F2013/Plants+I+-+Evolution+and+Diversity %2C+Nonvascular+Plants https://flexbooks.ck12.org/cbook/ck-12-middle-school-life-science2.0/section/7.2/primary/lesson/plants-adaptations-for-life-on-land-ms-ls https://blogs.scientificamerican.com/artful-amoeba/were-the-first-flowers-aquarium-plants/ http://www.bbc.com/earth/story/20141017-how-flowers-conquered-the-world https://cosmosmagazine.com/nature/plants/the-evolution-of-flowering-plants/ https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book %3A_General_Biology_(Boundless)/26%3A_Seed_Plants/26.1%3A_Evolution_of_Seed_Plants

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/26.1C%3A__Evolution_of_Angiosperms#:~:text=Fossil%20evidence%20indicates%20that %20flowering,traces%20of%20angiosperms%20are%20scarce. http://learn.pechnol.com/2018/04/06/really-wiped-dinosaurs-maybe-toxic-flowers/ (picture dino) https://www.google.com/url?sa=i&url=https%3A%2F%2Ftheconversation.com%2Frevealedthe-first-ever-flower-140m-years-ago-looked-like-a-magnolia81861&psig=AOvVaw2fYqUa6CwPfEMSWhPsLJH2&ust=1603244162421000&source=image s&cd=vfe&ved=0CAIQjRxqFwoTCPDrnfSFwuwCFQAAAAAdAAAAABAD https://theconversation.com/revealed-the-first-ever-flower-140m-years-ago-looked-like-amagnolia-81861 (magnolia) https://www.sciencedirect.com/science/article/pii/S0960982215014761#bib3 http://www.bbc.com/earth/story/20141017-how-flowers-conquered-the-world#:~:text=The %20first%20angiosperms%20must%20have,about%20200%20million%20years%20ago. Bell, P.R. and H, A.R. (2000) Green Plants: Their Origin and Diversity, Second Edition.London: Cambridge University Press Eckert, Christopher G., Marcel E. Dorken, and Spencer CH Barrett. "Ecological and evolutionary consequences of sexual and clonal reproduction in aquatic plants." Aquatic Botany 135 (2016): 46-61. Ponce de León, Inés, and Marcos Montesano. "Adaptation mechanisms in the evolution of moss defenses to microbes." Frontiers in plant science 8 (2017): 366. Boundless. “Boundless Biology.” Lumen, https://courses.lumenlearning.com/boundlessbiology/chapter/early-plant-life/.

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