Plant Life Cycles - Developmental Biology - NCBI Bookshelf
Alternation of generations (also known as metagenesis) is the type of life cycle that occurs in those plants and algae in the Archaeplastida and the Heterokontophyta that have distinct sexual haploid and asexual diploid stages. In these groups, a multicellular gametophyte, which is haploid with n The relationship between the sporophyte and gametophyte varies among. Gametophytes represent the haploid phase. Differences in size between gametophyte and sporophyte phases exist among vascular plants and. Specialized cells of the sporophyte will undergo meiosis and produce haploid spores. the sporophyte and the gametophyte and the relationship between them vary In other plants, such as ferns, both the gametophyte and sporophyte are.
Humans may also have selected the traits of some plants indirectly, as in the case of wild almonds.
Almonds contain a bitter compound called amygdalin that repels birds and other animals. Amygdalin breaks down into cyanide, 50 eating a large number of wild almonds can be fatal.
But mutations can reduce the level of amygdalin, making almonds sweet rather than bitter. Wild birds eat almonds from trees with such mutations.
According to one hypothesis, humans may have observed birds eating the almonds and then eaten the almonds themselves-ultimately using such seeds to grow trees that produced sweeter, less dangerous almonds.
Two of the world's most popular beverages come from tea leaves and coffee beans, and you can thank the tropical cacao tree for cocoa and chocolate.
Spices are derived from various plant parts, such as flowers cloves, saffronfruits and seeds vanilla, black pepper, mustard, cuminleaves basil, mint, sageand even bark cinnamon. Many seed plants, both gymnosperms and angiosperms, are sources of wood, which is absent in all living seedless plants. Wood consists of an accumulation of tough-walled xylem cells.
Wood is the primary source of fuel for much of the world, and wood pulp, typically derived from conifers such as fir and pine, is used to make paper. The resulting meiospores develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they only have one set of chromosomes. The mature gametophyte produces male or female gametes or both by mitosis.
Alternation of generations
The fusion of male and female gametes produces a diploid zygote which develops into a new sporophyte. This cycle is known as alternation of generations or alternation of phases. In flowering plantsthe sporophyte comprises the whole multicellular body except the pollen and embryo sac Bryophytes mossesliverworts and hornworts have a dominant gametophyte phase on which the adult sporophyte is dependent for nutrition.
The embryo sporophyte develops by cell division of the zygote within the female sex organ or archegoniumand in its early development is therefore nurtured by the gametophyte.Plant Reproduction in Angiosperms
Cleistocarpous sporophyte of the moss Physcomitrella patens Most algae have dominant gametophyte generations, but in some species the gametophytes and sporophytes are morphologically similar isomorphic. An independent sporophyte is the dominant form in all clubmosseshorsetailsfernsgymnosperms, and angiosperms that have survived to the present day. Early land plants had sporophytes that produced identical spores isosporous or homosporous but the ancestors of the gymnosperms evolved complex heterosporous life cycles in which the spores producing male and female gametophytes were of different sizes, the female megaspores tending to be larger, and fewer in number, than the male microspores.
During the Devonian period several plant groups independently evolved heterospory and subsequently the habit of endospory, in which the gametophytes develop in miniaturized form inside the spore wall.
By contrast in exosporous plants, including modern ferns, the gametophytes break the spore wall open on germination and develop outside it.
The Difference Between a Sporophyte and Gametophyte | Sciencing
The megagametophytes of endosporic plants such as the seed ferns developed within the sporangia of the parent sporophyte, producing a miniature multicellular female gametophyte complete with female sex organs, or archegonia.
The oocytes were fertilized in the archegonia by free-swimming flagellate sperm produced by windborne miniaturized male gametophytes in the form of pre-pollen. The resulting zygote developed into the next sporophyte generation while still retained within the pre- ovulethe single large female meiospore or megaspore contained in the modified sporangium or nucellus of the parent sporophyte.
The evolution of heterospory and endospory were among the earliest steps in the evolution of seeds of the kind produced by gymnosperms and angiosperms today.