practical 2

• Terrestrial plants have descended from a group of green algae called the Charophytes, explaining why the basic cell structure, photosynthetic apparatus and biochemistry, and basic patterns of cell division are virtually the same in the Charophytes and the terrestrial plants.
• Some would go so far as to say that terrestrial plants are nothing more than a special group of “land adapted green algae”.
• The terrestrial environment has imposed spectacular selective pressures on the terrestrial plants.
• Much of the evolutionary history of land plants can only be understood by examining the problems these plant groups must have overcome in order to survive in the terrestrial environment.
• Aquatic green algae and land plants need to be able to absorb light for photosynthesis.
• Aquatic green algae and land plants need to be able to absorb water from their surroundings and make it available to all the cells of the organism.
• Aquatic green algae and land plants need a supply of carbon dioxide and nitrogen compounds that are needed to build the cell’s organic molecules.
• Once the issues of photosynthesis, water absorption, and carbon dioxide and nitrogen compound supply have been dealt with, additional issues involving reproduction need to be considered.
• These include the protection of the delicate gamete forming structures, problems of gamete transfer, and survival of delicate young during reproduction.
• The wet, stable aquatic environment meant that algae didn’t need to protect their reproductive structures.
• However, elaborate protection systems for the delicate reproductive structures of the land plants had to evolve before successful reproduction could be ensured in the terrestrial environment.
• Alternation of Generations is a process where plants spend part of their life cycle as multicellular haploid gametophytes and part of their life cycle as multicellular diploid sporophytes.
• Sexual reproduction always includes the processes of meiosis and fertilization.
• Asexual reproduction never includes either of the above processes.
• Sexual reproduction always has a portion of the cycle in which all the cells of the organism are haploid (gametophyte generation), alternating with a portion of the cycle in which all cells of the organism are diploid (sporophyte generation)
• The sporophyte portion of the cycle always ends with the production of spores by meiosis (meiospores).
• The spores are produced in a structure known as sporangium (plural, sporangia).
• The gametophyte portion of the cycle terminates with the production of gametes by mitosis.
• The gametes are produced in a structure known as gametangium (plural, gametangia).
• There are two types of gametangia: the egg producing gametangia are known as archegonia (singular, archegonium), while the sperm producing ones are known as antheridia (singular, antheridium).
• Asexual reproduction always results in the formation of new individuals which are genetically identical to the parent organisms.
• Both asexual and sexual reproduction provide a means of increasing the population size.
• Sexual reproduction allows for variations in an organism's genotype and phenotype.
• If an organism is well suited to its environment, and if that environment is stable, then asexual reproduction may play an important role in maintaining the species.
• Genetic variation may be a short-term detriment in a stable environment.
• Sexual reproduction becomes more important in less stable environments.
• Changes in reproductive patterns parallel the evolution of plants towards a greater ability to withstand unstable terrestrial conditions.
• The basic plant groups seem to illustrate this evolution.
• Environmental pressures placed on reproductive adaptations evolved in plants to combat these pressures, the success of these adaptations, and the general trends in the evolution of plant reproductive systems should be considered when studying reproduction in the mosses, ferns, and seed plants.
• The concept of life cycles or life histories should be considered when studying reproduction in the mosses, ferns, and seed plants.
• The concept of alternation of generations should also be considered when studying reproduction in the mosses, ferns, and seed plants.
• Life cycles should be considered as a series of phases in the development of the organism.
• Each phase in a life cycle is a transition from the previous to the subsequent phase.
• Difficulty arises when one begins to examine, in detail, all the morphological changes that occur from one phase to another.
• In this case, the cycle itself may be ignored.
• Natural selection has worked on not only the morphological characteristics of theland plants, but also on the structures and strategies involved in sexual reproduction.This makes sense given the ultimate definition of the fitness of a phenotype (and ultimately a genotype). Fitness refers to the ability to produce the greatest number ofoffspring that live to reproduce themselves. Therefore, the more “fit” a species is, the more successful they will be.
• Survival of the gamete forming organs – On land, exposed to the dry air, the gametangia (gamete forming organs) on the gametophyte need an outer layer of protective cells toprevent them from drying out. This outer protective layer was not needed in the algae that lived underwater.
• Getting the gametes together – Transfer of the male gametes from the gametangia(antheridium) where they are made, to the female gametes (egg) inside the archegoniacan be a problem on land.
• The sperm in mosses and ferns are flagellated, and require free water for the sperm to swim through.
• The availability of water, such as rain, can severely limit the transfer of gametes and ultimately the success of reproduction in mosses and ferns.