Introduction
to Plant Life cycles
The life cycle of the
fern Ceratopteris richardii
In order to
understand plant life cycles, it is beneficial to some relevant terminology
before proceeding.
Diploid - A full set of
genetic material, consisting of homologous (paired) chromosomes. You inherit one half of each pair from your
mother (the maternal chromosomes) and one half of each pair from your father
(the paternal chromosomes). The diploid
human genome has 46 chromosomes (23 maternal and 23 paternal). Cells that are diploid are 2n.
Meiosis - The specialized cell division that takes
place when sex cells (sperm or egg cells) are produced. The members of each
homologous pair of chromosomes separate from each other so that each sex cell
receives only one member of each pair of
chromosomes.
Gametes – sex cells. Females produce eggs. Males produce sperm. Gametes are haploid.
Haploid - A single set of
chromosomes (half the full set of genetic material), present in the egg and sperm
cells of animals and in the egg and pollen cells of plants. Human beings have
23 chromosomes in their gametes. Cells
that are haploid are n.
Fertilization – The penetration of the egg by the sperm and
fusion of genetic materials to result in the development of an embryo.
zygote – a fertilized egg
Let us first
consider a life cycle that is fairly familiar to us – the human life
cycle.
Below is a
diagram of human reproduction, which is actually simpler than the life cycle of
plants.
Alternation of Generations Life Cycle of Plants
What is the
Alternation of Generations?
Plant sexual
reproduction is more complex than human reproduction. Plant sexual reproduction involves additional
steps between the key stages of meiosis & fertilization. Instead of directly producing sperm and eggs,
meiosis in plants produces spores that develop into gametophytes, which then
produce the sperm and eggs. In this way
the diploid sporophyptes and the haploid gametophytes alternate in producing
each other.
Sporophyte – (from the Greek words
meaning “spore-producing plant”)
The
diploid generation in the life cycle of a plant, and that produces haploid
spores by meiosis.
- In higher plants
it is the dominant phase of the plant life cycle.
- Each living cell
of the sporophyte contains two complete sets of chromosomes.
- The sporophyte
results from a union of haploid gametes (fertilization) that forms a
zygote. Mitotic divisions result in
growth.
- When mature, the
sporophyte produces spore-bearing structures.
Spore – A haploid, reproductive cell in plants that
is produced by meiosis and gives rise to the multicellular gametophyte by
mitotic division.
- The spore is a
unit of dispersal in the ferns; in this sense equivalent to seeds in
flowering plants.
Gametophyte –
(form
the Greek words meaning “gamete-producing plant”) The
haploid, gamete-producing stage of the plant life cycle.
- Gametophytes
produce gametes (either eggs or sperm or both) by means of mitosis.
- Each cell of a
gametophyte contains one complete set of chromosomes.
- In ferns, the
gametophyte is easily visible as a small green plant.
- In seed
producing plants the gametophyte generation is reduced to microscopic
proportions in the pollen grains and ovules.
The life cycle of the fern Ceratopteris
richardii
From sporophyte to spore germination
When
we think of ferns, we probably imagine something similar to the
What
we consider the leaf of the fern is actually a megaphyll, commonly referred to
as a frond. Fronds usually first appear tightly coiled at
their tips. These croziers, or “fiddleheads,” then unroll & expand, revealing the
blades. At maturity, the blades are
often divided into segments called pinnae
that are attached to a midrib, or rachis. A stalk,
or petiole, is usually present at the base.
When
the fronds have expanded, small, often circular, rust-colored patches of
powdery-looking material may appear on the lower surfaces of some or all of the
blades. These are clusters of sporangia. The sporangia are mostly found in numerous
discrete clusters called sori.
Most
of the sporangia are microscopic & stalked & look something like tiny
transparent baby rattles with a conspicuous row of heavy-walled brownish cells
along the edge. This row of cells, that
looks like a tiny millipede, is called an annulus. It functions in catapulting spores out of the sporangium with a
distinct snapping action influenced by moisture changes in the cells.
Sporocytes
undergo meiosis in the sporanagia, usually producing either 48 or 64 spores per
sporangium.
After
the spores have been flung out of their sporangia, they are dispersed by wind;
relatively few end up in habitats suitable for their survival. Such habitats include shady, wet ledges and
rock crevices or moist soil. (Spores
require water, light, and nutrients to germinate.) Those spores that germinate in favorable
locations produce little “Irish valentines,” or prothalli, as the green heart-shaped gametophytes are called. These structures often curl slightly at their
edges and may be 5 to 6 millimeters (1/4 inch) in diameter; they are visible
without a microscope.
Image modified
from: http://media-2.web.britannica.com/eb-media/59/72159-035-14F9199A.jpg
Fern Life Cycle
From Spore Development through
Gametophyte Development
Images
modified from the following: http://media-2.web.britannica.com/eb-media/59/72159-035-14F9199A.jpg
http://www.uwgb.edu/BIODIVERSITY/herbarium/pteridophytes/fern_sorus01.jpg
Stern, Kingsley R. et al. Introductory
Plant Biology: 11th ed. McGraw-Hill 2008.
http://phobos.ramapo.edu/~spetro/Slides/_fern_proth40x.jpg
The
life cycle of the fern Ceratopteris
richardii
From gametophyte to sporophyte
development
The prothallus is the green-heart shaped gametophyte. On the underside of the prothallus, in the
central area of the lower side, are the rhizoids. Rhizoids are cells that anchor the plant and
absorb water. Interspersed among the
rhizoids are the antheridia (male
sex organs). Antheridia look like small
clusters of grapes (they are spherical & often elevated above the surface
on short stalks). A single antheridium
may produce from 32 to several hundred sperm,
each with few to many flagella.
The archegonium (female sex organs), also found on the underside of the
prothallus, usually grow closer to the growing notch (apical notch) of the
heart-shaped gametophyte. The archegonia
are somewhat flask shaped, with curving necks that protrude slightly above the
surface. They look like small, green
volcanoes. Within the archegonium, one egg will be produced.
Both the egg and the sperm are
haploid (containing half the genetic material of the sporophyte). The antheridium are stimulated to release the
sperm by the splashing of raindrops. The
sperm are coiled in shape & use their flagella to swim through a thin layer
of water toward the egg. The egg attracts
the sperm with chemicals known as pheromones. Fertilization
of an egg takes place within an archegonium.
Only one zygote develops into
a young sporophyte on any prothallus, regardless of the number of eggs that may
be fertilized. This sporophyte usually
has smaller, simpler fronds during its first growing season, but typical
full-sized fronds grow from the persisting rhizomes
in succeeding years.
There are actually two distinct
gametophyte forms in fern. The larger
heart-shaped gametophytes have both male and female sex organs & are
referred to as hermaphroditic
gametophytes. Among these larger
hermaphroditic gametophytes, there should also be numerous small, tongue-shaped
male plants. Males have many small,
round structures, about the size of a vegetative cell, which bulge out form the
surface. These are male sex organs, or
antheridia. Each antheridium produces 16
male gametes or sperm. Males develop in
response to the presence of a special chemical (antheridiogen) in the medium that is secreted by developing
hermaphroditic gametophytes.
Images drawn by
Scarlet Estlack
Fern Life Cycle
From Spore Development through
Gametophyte Development
Images
modified from: http://legacy.lclark.edu/~seavey/images%20/fern-life-cycle.jpg
http://www.esu.edu/~milewski/intro_biol_two/lab_2_moss_ferns/images/fern_life_cycle.gif
