Chapter 30 The Evolution of Seed Plants Seed Plants have altered the landscapes around the world They can colonize disaster areas such as areas that suffered from a volcanic eruption Seed Plants have become the dominant producers on land and make up the vast majority of plant biodiversity 30 1 SEEDS AND POLLEN GRAINS ARE KEY ADAPTATIONS FOR LIFE ON LAND All seed plants have reduced gametophytes heterospory ovules and pollen These adaptations have allowed for plants to deal with droughts UV radiation and much more Seeds also freed plants from requiring water for fertilization ADVATNAGES OF REDUCED GAMETOPHYTES The evolutionary trend of gametophyte reduction continued further in vascular plant lineage that led to seed plants The gametophytes of most seed plants are microscopic and their evolutionary shrinkage allowed for tiny gametophytes to develop inside the sporangia of the parent sporophyte and protect it from the environment UV radiation drought and obtain nutrients from parental sporophytes This figure shows the various gametophyte sporophyte relationships in different plants HETEROSPORY THE RULE AMONG SEED PLANTS Seed plants or their ancestors became heterosporous producing megasporangia female gametophytes and microsporangia male gametophytes Each megasporium has one megaspore and each microsporangium has many microspores OVULES AND PRODUCTION OF EGGS Although a few species of seedless plants are heterosporous seed plants are unique in retaining the megasporanium within the parent sporophyte INTEGUMENT sporophyte tissue envelops and protects the megasporanium Gymnosperms have 1 one integument while angiosperms have 2 An ovule consists of a megasporanium megaspore and their integument s Inside each ovule an egg is produced from a megaspore POLLEN AND PRODUCTION OF SPERM A microspore develop into a pollen grain that consists of a male gametophyte enclosed within the pollen wall The male gametophyte is in the pollen grain Sporpollenin protects the pollen wall as it is transported Pollination is the transfer of pollen to the part of the seed that contains the ovule Pollen eliminates the need for water for fertilization and contributed to the colonization of land THE EVOLUTIONARY ADVATNAGE OF SEEDS IF sperm fertilizes an egg of a seed plant the zygote grows into a sporophyte embryo Until the birth of seeds spores were the only protective stage in any plant life cycle The advantages of Seeds over Spores include multi cellularity and an embryo protected by a layer of tissue called the seed coat Seeds have longer life times than Spores and can remain dormant for a long time 30 2 GYMNOSPERMS BEAR NAKED SEEDS TYPICALLY ON CONES Extant seed plants form 2 sister clades gymnosperms and angiosperms Gymnosperms have naked seeds exposed on sporophylls that usually form cones strobili Angiosperms are enclosed in chambers that mature into fruit Most gymnosperms are cone bearing plans called conifers The pine tree is the sporophyte its sporangia are located on scale like structures packed densely in cones The two types of spores are produced by small pollen cones and large ovulate cones Cells called microsporocytes undergo meiosis producing haploid microspores which develop into a pollen grain In ovulate cones MEGASPOROCYTES undergo meiosis and produce haploid megaspores inside the ovule and the survivors develop into female gametophytes which are retained in the sporangia EARLY SEED PLANTS AND RISE OF GYMNOSPERMS The origin of characteristics found in pines and other living seed plants date back to the late Devonian period Fossils reveal some plants had acquired features that are also present in seed plants such as megaspores and microspores The early gymnosperms lived in moist Carboniferous ecosystems still dominated by lycophytes horsetails ferns and other seedless vascular plants As conditions became drier Gymnosperms prevailed because they were more suited to drier climates Gymnosperms dominated terrestrial life through the Mesozoic era and were a key food source for Dinosaurs Late in the Mesozoic angiosperms began to replace gymnosperms in some ecosystems GYMNOSPERMS DIVERSITY Although angiosperms dominate most terrestrial ecosystems gymnosperms are very import Of the 10 plant phyla four are gymnosperms They are Cycadophyta Gingkophyta Gnetophyta and Coniferophyta Phylum Cycadophyta Have large cones and palmlike leaves Cycads have flagellated sperm indicating their descent from seedless vascular plants that had motile sperm Phylum Gnetophyta Includes plants in three genera Gnetum Ephedra and Welwitschia Grouped together based on molecular data Phylum Gingophyta Gino biloba is the only surviving species of this phylum have flagellated sperm Phylum Coniferophyta largest gymnosperm phyla Most have woody cones Have needle like leaves or scale like leaves Retain leaves throughout the year 30 3 REPRODUCTIVE ADAPATIONS OF ANGIOSPERMS INCLUDE FLOWERS AND FRUITS Angiosperms have seeds in fruits They are the most diverse and widespread of all plants CHACTETERISTICS OF ANGIOSPERMS All angiosperms are classified in a single phylum Anthrophyta Fruits and flowers are the two key adaptations Flowers The flower is a unique angiosperm structure specialized for sexual reproduction Pollination is more direct than wind pollination of most gymnosperms because insects and animals transfer pollen from one flower to another Angiosperms in dense areas and near forests are usually wind pollinated A flower is a specialized shoot with SEPALS enclose flower an interior PETAL attracts pollinators STAMENS produce microspores consist of FILAMENT terminal sac and an ANTHER produce pollen CARPELS make megaspores STIGMA which is a stick that receives pollen The STYLE leads from the stigma to the overy Flowers with all 4 organs are COMPLETE FLOWERS while those lacking one or more of the organs are INCOMPLETE FLOWERS FRUITS As seeds develop from ovules after fertilization the ovary wall thickens and the ovary matures into a fruit Fruits protect seeds and aid in their dispersal Fleshy fruits are due to the ovary becoming soft during ripening Dry fruits split open at maturity to release seeds Various adaptations of fruits and seeds help disperse seeds Such adaptations include Propellers or parachutes in dandelions and maples Vividly colored fruit attract predators to each the fruit which is digested but the Fruit modified as burrs to be carried by animals seeds are left unharmed defecation results in a deposit of seeds and