INTRODUCTION
TO LIFE CYCLES
The complete series of events from zygote formation to gamete production constitutes the plant life cycle. It includes all the processes involved from with the growth, development, and reproduction of plant species.
There are three different plant life cycles: haploid (1n),
diploid (2n), and the more common haploid-diploid (1n-2n).
A haploid organism consists of a multicellular structure of cells that contain only one set of chromosomes, whereas, a diploid organism's multicellular stage contains two sets of chromosomes.
Of the haplid organisms, there are some fungi that go through a dikaryotic phase and some plants and fungi (note: fungi are now placed in a separate kingdom from plants, the Fungi kingdom) that lack a dikaryotic phase.
Since we are concentrating on the life histories of plants, we will focus on plants that lack the dikaryotic phase. In a 1n-2n life cycle, there are two multicellular generations, a haploid (1n gametophyte) alternating with a diploid (2n sporophyte) stage.
Moreover, there is an alternation of either similar or dissimilar generations. In the first case, the 1n gametophyte is morphologically identical to the 2n sporophyte, except for the reproductive structures, while in the latter case, the two generations are morphologically different.
For the alternation of dissimilar generations, there are three possible relationships between the gametophyte and the sporophyte: 1. both are totally independent of each other at maturity (each being free-living), 2. the sporophyte is dominant, or 3. the gametophyte is dominant.
Moreover, there are also variations within each of these life histories.
Asexual, or vegetative, reproduction may also occur within all three life cycles.
Sexual production is characterized by syngamy (fertilization) which involves the fusion of 1n gametes to form a zygote and is followed by meiosis in which a 2n structure divides into four different 1n cells.
Asexual reproduction, on the other hand, does not involve these two processes.
As for the evolutionary history of these life cycles, haploid alga were predominant prior to the origin of land plants (vascular plants and bryophytes).
However, the ancestor of all land plants was haploid-diploid.
A vascular plant, Magnolia.
ALL TERMS IN BOLD ARE DEFINED IN THE GLOSSARY.
