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.

 

zygotea 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.

 

Human Reproduction
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 Boston ferns that we see for sale in local grocery stores and nurseries.  In all ferns, this conspicuous phase of the plant’s life cycle is the sporophyte.   A fern sporophyte consists of the fronds, a stem in the form of a rhizome, and adventitious roots that arise along the rhizome.

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.

fern sporophyte

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

FernLifeCycle pict

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

http://www.biologie.uni-hamburg.de/b-online/library/cfern/cfern.bio.utk.edu/images/gallery/germinatingspores.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.

Gametophyte

Images drawn by Scarlet Estlack


Fern Life Cycle

From Spore Development through Gametophyte Development

FernLifeCycle 2 pict

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