This Document was created for a Botany class I taught at a different universityseveral years ago. It is a summary of information on the major groups of landplants (Embryophyta) as well as some explanatory information about details ofplant structure and life cycles.Important – I do not expect anyone to read this entire document. I am providing itto you for background information on the various groups which you can use to helpyou prepare your report. You can also use it to get a more complete explanation ofanything that is briefly mentioned in the lab.Kevin DixonSexual Life Cycles. Sexual reproduction is another major evolutionary innovation of theeukaryotes. The selective advantage of sexual reproduction to an individual organism isstill unclear but it does allow for much greater genetic diversity within populations andhence at least the potential for more rapid evolution. Although this is a fascinating topicwe 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 ahaploid and part of its cycle as diploid. In every sexual life cycle at some point a diploidnucleus undergoes meiosis and produces four haploid nuclei (point A). At some otherpoint in the life cycle two haploid nuclei fuse to form one diploid nuclei (point B). Whathappens between point A and point B (and between point B and the next point A) istremendously variable among eukaryotes. It is possible to divide the life cycles into threegroups which are outlined below. The categories are simplifications and not allorganisms fit neatly in one of them.1. Gametic Meiosis. The only haploid cells are the gametes which do not reproduce orgrow into multicellular entities. The gametes’ only function is to fuse and form diploidcells (zygotes). The diploid phase persists over a prolonged period of time, feeds, mayreproduce asexually and/or forms a multicellular organism which eventually producesmore gametes through meiosis. This life cycle is found in animals. It name is derivedfrom 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 cellundergoes meiosis to produce haploid cells which feed, reproduce asexually, and/or growinto multicellular entities. Eventually the haploid stage will produce gametes (throughmitosis rather than meiosis) which unite to form a new zygote. This type of life cycle isfound in most green algae and in most fungi (usually in a highly modified form). It’sname is derived from the fact that the zygote is the stage in the life cycle that undergoesmeiosis.3. Sporic Meiosis. Both the diploid and haploid phases of the life cycle are long-lastingand generally multicellular. The diploid phase (called the sporophyte) produces haploidspores through meiosis. Each spore grows into a haploid organism called a gametophyte.The gametophyte produces gametes through mitosis. Two gametes unite to produce adiploid zygote which grows into a new sporophyte. This type of life cycle is typical ofland plants (Embryophytes). It’s name is derived from the fact that spores are the productof 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 relativesizes of the gametophyte and the sporophyte. If the gametophyte and sporophyte areessentially identical in morphology then the life cycle is said to be isomorphic (e.g. somegreen algae). If the gametophyte and the sporophyte are different in size then the lifecycle 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 betweengametes and zygotes. Generally one of the haploid or the diploid stage is longer-lastingand/or more ecologically active (e.g. feeds) than the other and unicellular organisms areclassified as having either zygotic or gametic meiosis.The selective advantages of different types of life cycles is a fascinating but sparinglyinvestigated subject.*Embryophyta. The 'land plants' are the most diverse group of photosyntheticorganisms and the dominant group on land. As a group they are characterized by havingheteromorphic sporic meiosis, multicellular gametangia (gamete producing structures)enclosed within sterile jacket cells, embryos (a juvenile multicellular stage enclosedwithin the maternal gametophyte), and a large nonmotile female gamete and a smallmotile female gamete (oogamous). The Embryophyta consists of of three groups ofnonvascular plants: the Anthocerophyta (hornworts), the Hepatophyta (liverworts), andthe Bryophyta (mosses) as well as the Tracheophyta (vascular plants). Relationshipsamong these four groups are uncertain and will be discussed at the end of the section onthe Bryophyta.Invasion of the Land. Although the embryophytes are not the only photosyntheticorganisms to live on land and many of the embryophytes live in the water the title 'landplants' is appropriate. Algae that live on land are only found in moist microhabitats or aredormant except when it is wet. Life on land has a number of advantages over the aquaticenvironment but also some additional challenges. Air contains a much greaterconcentrations of oxygen and carbon dioxide than water. In water light is only availableat all relatively near the surface and the intensity of light drops off rapidly as it passesthrough water. Light is much more available in the air. However, in water plants arebuoyant and and can float. Also water and nutrients are available through the aquaticenvironment and specialized structures are not required for nutrient acquisition. On landwater and nutrients are generally unavailable in the atmosphere although they can beabundant in the soil.The nature of the terrestrial environment requires that land plants have specializedstructures 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 sudividedinto stems and leaves. Leaves actually perform photosynthesis and gas exchange whilestems provide support to raise leaves above the substrate in order to