Plant Diversity II.pptx

Plant Diversity II: Seed Vascular Plants  Plant Diversity II: Seed Vascular Plants I. General Characteristics II. Gymnosperms III. Angiosperms IV. Ecological Value of Angiosperms  Learning Objectives 1. Describe the following general characteristics of seed-vascular pl...

Plant Diversity II: Seed Vascular Plants  Plant Diversity II: Seed Vascular Plants I. General Characteristics II. Gymnosperms III. Angiosperms IV. Ecological Value of Angiosperms  Learning Objectives 1. Describe the following general characteristics of seed-vascular plants: seeds and seed development, reduced gametophytes, heterospores, pollen, ovules 2. Explain the characteristics of gymnosperms 3. Identify the gymnosperm phyla and examples of plants in each phylum 4. Organize the sequence of developmental stages in the life cycle of a pine tree Learning Objectives (contd.) 5. Describe the structure and function of a flower 6. Describe the structure and function of a fruit 7. Identify the characteristics of the Phylum Anthophyta and its sub- divisions 8. Organize the sequence of developmental stages in the life cycle of an angiosperm  I. General Characteristics A. Seeds 1. Seed plants originated about 360 million years ago 2. A seed consists of an embryo and nutrients surrounded by a protective coat 3. Domestication of seed plants had begun by 8,000 years ago and allowed for permanent settlements B. Reduced Gametophyte The gametophytes of seed plants develop within the walls of spores that are retained within tissues of the parent sporophyte. C. Heterospory Seed plants produce two types of spores: Megasporangia produce megaspores that give rise to female gametophytes Microsporangia produce microspores that give rise to male gametophytes D. Ovule and egg production An ovule consists of a megasporangium, megaspore, and one or more protective integuments Gymnosperm megaspores have one integument Angiosperm megaspores usually have two integuments E. Pollen and sperm production Microspores develop into pollen grains, which contain the male gametophytes Pollination is the transfer of pollen to the part of a seed plant containing the ovules Pollen eliminates the need for a film of water and can be dispersed great distances by air or animals If a pollen grain germinates, it gives rise to a pollen tube that discharges sperm into the female gametophyte within the ovule F. Seed Development A seed develops from the whole ovule A seed is a sporophyte embryo, along with its food supply, packaged in a protective coat – They may remain dormant for days to years, until conditions are favorable for germination – They may be transported long distances by wind or animals  II. Gymnospems Gymnosperms means “naked seeds” The seeds are exposed on sporophylls that form cones Angiosperm seeds are found in fruits, which are mature ovaries A. Phylum Cycadophyta Individuals have large cones and palmlike leaves These thrived during the Mesozoic, but relatively few species exist today B. Phylum Ginkgophyta This phylum consists of a single living species, Ginkgo biloba It has a high tolerance to air pollution and is a popular ornamental tree C. Phylum Gnetophyta This phylum comprises three genera Species vary in appearance, and some are tropical whereas others live in deserts D. Phylum Coniferophyta This phylum is by far the largest of the gymnosperm phyla Most conifers are evergreens and can carry out photosynthesis year round E. Conifer Life Cycle Three key features of the gymnosperm life cycle are – Dominance of the sporophyte generation – Development of seeds from fertilized ovules – The transfer of sperm to ovules by pollen The pine tree is the sporophyte and produces sporangia in male and female cones Small cones produce microspores called pollen grains, each of which contains a male gametophyte The familiar larger cones contain ovules, which produce megaspores that develop into female gametophytes  III. Angiosperms Angiosperms are seed plants with reproductive structures called flowers and fruits They are the most widespread and diverse of all plants All angiosperms are classified in a single phylum, Anthophyta, from the Greek anthos for flower Angiosperms have two key adaptations – Flowers – Fruits A. Flowers The flower is an angiosperm structure specialized for sexual reproduction Many species are pollinated by insects or animals, while some species are wind-pollinated A flower is a specialized shoot with up to four types of modified leaves – Sepals, which enclose the flower – Petals, which are brightly colored and attract pollinators – Stamens, which produce pollen – Carpels, which produce ovules B. Fruits 1. A fruit typically consists of a mature ovary but can also include other flower parts 2. Fruits protect seeds and aid in their dispersal 3. Mature fruits can be either fleshy or dry 4. Various fruit adaptations help disperse seeds 5. Seeds can be carried by wind, water, or animals to new locations C. Angiosperm Life Cycle 1. The flower of the sporophyte is composed of both male and female structures 2. Male gametophytes are contained within pollen grains produced by the microsporangia of anthers 3. The female gametophyte, or embryo sac, develops within an ovule contained within an ovary at the base of a stigma 4. Most flowers have mechanisms to ensure cross- pollination between flowers from different plants of the same species 5. A pollen grain that has landed on a stigma germinates and the pollen tube of the male gametophyte grows down to the ovary 6. The ovule is entered by a pore called the micropyle 7. Double fertilization occurs when the pollen tube discharges two sperm into the female gametophyte within an ovule 8. One sperm fertilizes the egg, while the other combines with two nuclei in the central cell of the female gametophyte and initiates development of food-storing endosperm 9. The triploid endosperm nourishes the developing embryo 10. Within a seed, the embryo consists of a root and two seed leaves called cotyledons D. Angiosperm Diversity 1. Angiosperms comprise more than 250,000 living species 2. Previously, angiosperms were divided into two main groups 1. Monocots (one cotyledon) 2. Dicots (two dicots) 3. DNA studies suggest that monocots form a clade, but dicots are polyphyletic Basal Angiosperms Three small lineages constitute the basal angiosperms These include Amborella trichopoda, water lilies, and star anise Magnoliids Magnoliids include magnolias, laurels, and black pepper plants Magnoliids are more closely related to monocots and eudicots than basal angiosperms Monocots More than one- quarter of angiosperm species are monocots Eudicots More than two- thirds of angiosperm species are eudicots  IV. Ecological Value of Angiosperms A. Plants and Animals Animals influence the evolution of plants and vice versa – For example, animal herbivory selects for plant defenses – For example, interactions between pollinators and flowering plants select for mutually beneficial adaptations B. Plants and People No group of plants is more important to human survival than seed plants Plants are key sources of food, fuel, wood products, and medicine Our reliance on seed plants makes preservation of plant diversity critical C. Ecological Threats to Plants Destruction of habitat is causing extinction of many plant species In the tropics 55,000 km2 are cleared each year At this rate, the remaining tropical forests will be eliminated in 200 years Loss of plant habitat is often accompanied by loss of the animal species that plants support At the current rate of habitat loss, 50% of Earth’s species will become extinct within the next 100–200 years