Plant Life Cycle -
Alternation of Generations
Plants are able to reproduce
by what is known as alternation of generations. Alternation of generations
describes a plant's life cycle as it alternates between a sexual phase or
generation and an asexual phase. The sexual generation in plants produces
gametes, or sex cells, and is called the gametophyte generation. The asexual
phase produces spores and is called the sporophyte generation. Each generation
develops from the other, continuing the cyclical process. Protist organisms
including algae also exhibit this type of life cycle.
Plant and Animal
Reproduction
Plants and some animals
are capable of reproducing both asexually and sexually. In asexual
reproduction, the offspring are an exact duplicate of the parent. Types of
asexual reproduction commonly seen in both plants and animals include
parthenogenesis (offspring develops from an unfertilized egg), budding
(offspring develops as a growth on the parent's body), and fragmentation
(offspring develops from a part or fragment of the parent). Sexual reproduction
involves the uniting of haploid cells (cells containing only one set of
chromosomes) to form a diploid (containing two chromosome sets) organism.
In multicellular
animals, the life cycle consists of a single generation. The diploid organism
produces haploid sex cells by meiosis. All other cells of the body are diploid
and produced by mitosis. A new diploid organism is created by the fusion of
male and female sex cells during fertilization. The organism is diploid and
there is no alternation of generations between haploid and diploid phases.
In plant multicellular
organisms, life cycles vacillate between diploid and haploid generations. In
the cycle, the diploid sporophyte phase produces haploid spores via meiosis. As
haploid spores grow by mitosis, the multiplied cells form a haploid gametophyte
structure. The gametophyte represents the haploid phase of the cycle. Once
mature, the gametophyte produces male and female gametes. When haploid gametes
unite, they form a diploid zygote. The zygote grows via mitosis to form a new
diploid sporophyte. Thus unlike in animals, plant organisms can alternate
between diploid sporophyte and haploid gametophyte phases.
Vascular and
Non-vascular Plants
Alternation of
generations is seen in both vascular and non-vascular plants. Vascular plants
contain a vascular tissue system that transports water and nutrients throughout
the plant. Non-vascular plants do not have this type of system and require
moist habitats for survival. Non-vascular plants include mosses, liverworts,
and hornworts. These plants appear as green mats of vegetation with stalks
protruding from them. The primary phase of the plant life cycle for
non-vascular plants is the gametophyte generation. The gametophyte phase
consists of green mossy vegetation, while the sporophtye phase consists of
elongated stalks with a sporangium tip that encloses the spores.
The primary phase of
the plant life cycle for vascular plants is the sporophtye generation. In
vascular plants that do not produce seeds, such as ferns and horsetails, the sporophtye
and gametophyte generations are independent. In ferns for example, the leafy
fronds represent the mature diploid sporophyte generation. The sporangia on the
undersides of the fronds produce the haploid spores, which germinate to form
the haploid fern gametophytes (prothallia). These plants thrive in damp
environments as water is required for the male sperm to swim toward and
fertilize the female egg.
Vascular plants that
produce seeds are not necessarily dependent upon moist environments to reproduce.
The seeds protect the developing embryos. In both flowering plants and
nonflowering plants (conifers), the gametophyte generation is totally dependent
upon the dominant sporophtye generation for survival. In flowering plants, the
reproductive structure is the flower. The flower produces both male microspores
and female megaspores. The male microspores are contained within pollen and are
produced in the plant stamen. They develop into the male gametes or sperm. The
female megaspores are produced in the plant ovary. They develop into female
gametes or eggs. During pollination, pollen is transfered via wind, insects or
other animals to the female part of a flower. Male and female gametes unite in
the ovary and develop into a seed, while the ovary forms the fruit. In
conifers, pollen is produced in male cones and eggs are produced in female
cones.
Sources:
https://www.thoughtco.com/plant-life-cycle-alternation-of-generations-373612
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