This Document was created for a Botany class I taught at a different university several years ago It is a summary of information on the major groups of land plants Embryophyta as well as some explanatory information about details of plant structure and life cycles Important I do not expect anyone to read this entire document I am providing it to you for background information on the various groups which you can use to help you prepare your report You can also use it to get a more complete explanation of anything that is briefly mentioned in the lab Kevin Dixon Sexual Life Cycles Sexual reproduction is another major evolutionary innovation of the eukaryotes The selective advantage of sexual reproduction to an individual organism is still unclear but it does allow for much greater genetic diversity within populations and hence at least the potential for more rapid evolution Although this is a fascinating topic we will not consider it in detail until later in the course One consequence of sexual reproduction is that a species spends part of its life cycle as a haploid and part of its cycle as diploid In every sexual life cycle at some point a diploid nucleus undergoes meiosis and produces four haploid nuclei point A At some other point in the life cycle two haploid nuclei fuse to form one diploid nuclei point B What happens between point A and point B and between point B and the next point A is tremendously variable among eukaryotes It is possible to divide the life cycles into three groups which are outlined below The categories are simplifications and not all organisms fit neatly in one of them 1 Gametic Meiosis The only haploid cells are the gametes which do not reproduce or grow into multicellular entities The gametes only function is to fuse and form diploid cells zygotes The diploid phase persists over a prolonged period of time feeds may reproduce asexually and or forms a multicellular organism which eventually produces more gametes through meiosis This life cycle is found in animals It name is derived from the fact that gametes are the direct product of meiosis 2 Zygotic Meiosis The only diploid cell in the life cycle is the zygote This cell undergoes meiosis to produce haploid cells which feed reproduce asexually and or grow into multicellular entities Eventually the haploid stage will produce gametes through mitosis rather than meiosis which unite to form a new zygote This type of life cycle is found in most green algae and in most fungi usually in a highly modified form It s name is derived from the fact that the zygote is the stage in the life cycle that undergoes meiosis 3 Sporic Meiosis Both the diploid and haploid phases of the life cycle are long lasting and generally multicellular The diploid phase called the sporophyte produces haploid spores through meiosis Each spore grows into a haploid organism called a gametophyte The gametophyte produces gametes through mitosis Two gametes unite to produce a diploid zygote which grows into a new sporophyte This type of life cycle is typical of land plants Embryophytes It s name is derived from the fact that spores are the product of meiosis It is also known as Alternation of Generations referring to the sporophyte diploid and gametophyte haploid generations Organisms exhibiting sporic meiosis can be further broken down based on the relative sizes of the gametophyte and the sporophyte If the gametophyte and sporophyte are essentially identical in morphology then the life cycle is said to be isomorphic e g some green algae If the gametophyte and the sporophyte are different in size then the life cycle is said to be heteromorphic with either a dominant sporophyte e g vascular plants or a dominant gametophyte e g non vascular plants It is also possible to have a life cycle of a unicellular organism that alternates between gametes and zygotes Generally one of the haploid or the diploid stage is longer lasting and or more ecologically active e g feeds than the other and unicellular organisms are classified as having either zygotic or gametic meiosis The selective advantages of different types of life cycles is a fascinating but sparingly investigated subject Embryophyta The land plants are the most diverse group of photosynthetic organisms and the dominant group on land As a group they are characterized by having heteromorphic sporic meiosis multicellular gametangia gamete producing structures enclosed within sterile jacket cells embryos a juvenile multicellular stage enclosed within the maternal gametophyte and a large nonmotile female gamete and a small motile female gamete oogamous The Embryophyta consists of of three groups of nonvascular plants the Anthocerophyta hornworts the Hepatophyta liverworts and the Bryophyta mosses as well as the Tracheophyta vascular plants Relationships among these four groups are uncertain and will be discussed at the end of the section on the Bryophyta Invasion of the Land Although the embryophytes are not the only photosynthetic organisms to live on land and many of the embryophytes live in the water the title land plants is appropriate Algae that live on land are only found in moist microhabitats or are dormant except when it is wet Life on land has a number of advantages over the aquatic environment but also some additional challenges Air contains a much greater concentrations of oxygen and carbon dioxide than water In water light is only available at all relatively near the surface and the intensity of light drops off rapidly as it passes through water Light is much more available in the air However in water plants are buoyant and and can float Also water and nutrients are available through the aquatic environment and specialized structures are not required for nutrient acquisition On land water and nutrients are generally unavailable in the atmosphere although they can be abundant in the soil The nature of the terrestrial environment requires that land plants have specialized structures for specific functions Land plants are divided into two fundamental sections roots and shoots Roots are specialized for acquiring water and nutrients from the soil Shoots are specialized for gas exhange and photosynthesis Shoots are further sudivided into stems and leaves Leaves actually perform photosynthesis and gas exchange while stems provide support to raise leaves above the substrate in order to maximize exposure to sunlight Nonvascular plants lack true roots and the lack of a vascular system restricts