Chapter 30 Plant Diversity 2 The evolution of seed plants 1 Chapter 30 Plant Diversity 2 The Evolution of Seed Plants Transforming the World A seed consists of an embryo and its food supply surrounded by a protective coat Concept 30 1 Seeds and pollen grains are key adaptations for life on land The following are common to all seed plants reduced gametophytes heterospory ovules and pollen Novel adaptations also freed seed plants from requiring water for fertilization enabling reproduction to occur under a broader range of conditions than in seedless plants Advantages of Reduced Gametophytes Mosses and other bryophytes have life cycles dominated by gametophytes Gametophytes of seed plants are microscopic Ferns and other seedless vascular plants have sporophyte dominated life cycles Gametophytes of seedless vascular plants are visible Tiny gametophytes can develop from spores retained within the sporangia of the parental sporophyte The moist reproductive tissues of the sporophyte shield the gametophytes from UV radiation and protect them from drying out Heterospory the rule among seed plants Most seedless plants are homosporous they produce one kind of spore which usually gives rise to a bisexual gametophyte Megasporangia produce megaspores that give rise to female gametophytes Microsporangia produce microspores that give rise to male gametophytes Ovules and Production of Eggs Seed plants are unique in retaining the megasporangium within the parent sporophyte Integument is a layer of sporophyte tissue that envelops and protects the megasporangium Gymnosperms are surrounded by one integument whereas those in angiosperms usually have two Pollen and Production of Sperm wall Ovule is the entire structure consisting of the megasporangium megaspore and their integument s Inside each ovule a female gametophyte develops from a megaspore and produces one or more eggs A microspore develops into a pollen grain that consists of a male gametophyte enclosed within the pollen The outer layer of the pollen wall is composed to molecules secreted by sporophyte cells The tough pollen wall contains the polymer sporopollenin that protects a pollen grain as it is transported form the parent plant by wind or animal Pollination is the transfer of pollen to the part of a seed plant that contains the ovules Remember that nonvascular plants and seedless vascular plants such as ferns free living gametophytes release flagellated sperm that swim through a film of water to reach eggs The sperm of some gymnosperm species retain the ancient flagellated condition but flagella have been lost in the sperm of most gymnosperms and all angiosperms Chapter 30 Plant Diversity 2 The evolution of seed plants 2 The Evolutionary Advantage of Seeds If a sperm fertilizes an egg of a seed plant the zygote grows into a sporophyte embryo Spores were the main way that mosses ferns and other seedless plants spread over earth for millions Spores are usually single celled whereas seeds are multicellular consisting of an embryo protected by a layer Seeds can remain dormant for days months or even years after being released from the parent plant of years of tissues the seed coat whereas most spores have shorter lifetimes Seeds have a supply of stored food Most seeds land close to their parent sporophyte plant but some are carried long distances by wind or Concept 30 1 Summary Seeds and pollen grains are key adaptations for life on land animal Five derived traits of seed plants Reduced Gametophytes Heterospory Microspore gives rise to male gametophyte Megaspore Gives rise to a female gametophyte Microscopic male and female gametophytes n are nourished and protected by the sporophytes 2n Ovules Ovule gymnosperm Integument 2n Megaspore n Megasporangium 2n Pollen Seeds Seed coat Food supply Embryo Pollen grains make water unnecessary for fertilization Seeds survive better than unprotected spores can be transported long distances Concept 30 2 Gymnosperms bear naked seeds typically on cones Gymnosperm Evolution Gymnosperms have naked seeds that are not enclosed in ovaries Their seeds are exposed on modified leaves sporophylls that usually form cones strobili Angiosperm seeds are enclosed in fruits which are mature ovaries Such transitional species of seedless vascular plants are sometimes called progymnosperms As the Carboniferous period gave way to the Permian markedly drier climatic conditions favored the spread of gymnosperms Chapter 30 Plant Diversity 2 The evolution of seed plants 3 The lycophytes horsetails and ferns that dominated the Carboniferous swamps were largely replaced by gymnosperms which were more suited for drier climate Some gymnosperms were particularly well suited to arid conditions because of the thick cuticles and relatively small surface areas of their needle shaped leaves Life changed profoundly as gymnosperms dominated terrestrial ecosystems throughout much of the Mesozoic serving as the food supply for giant herbivorous dinosaurs Although angiosperms now dominate most terrestrial ecosystems many gymnosperms remain an important part of earth s flora Vast regions of northern latitudes are covered by forests of cone bearing gymnosperms called conifers which include spruce pine fir and redwood Four types of gymnosperms Phylum Cycadophyta The second largest group of gymnosperms after conifers Phylum Ginkgophyta Deciduous fanlike leaves Popular ornamental tree in cities because it tolerates air pollution well Fleshy seeds smell rancid as they decay Phylum Gnetophyta Welwitschia Phylum Coniferophyta Plants in the phylum Gnetophyta called gnetophytes consist of three genera Gentum Ephedra and The largest of the gymnosperm phyla Many are large trees and dominate vast forest regions Most are evergreens they retain their leaves throughout the year The life cycle of a pine a closer look Seed plant evolution has included three key reproductive adaptations The increasing dominance of the sporophyte The advent of the seed as a resistant dispersible stage in the life cycle The appearance of pollen as an airborne agent that brings gametes together The life cycle of a pine 1 In most conifer species each tree has both ovulate and pollen cones 2n 2 Microsporocytes divided by meiosis producing haploid microspores A microspore develops into a pollen grain a male gametophyte enclosed within the pollen wall 2n 3 An ovulate cone scale has two ovules each containing megasporangium 2n 4 Pollination occurs when a pollen grain reaches the