Each plant is characterized by one of the three life histories: haploid (1n), diploid (2n), or the most common haploid-diploid. Within each of these three types, there are also variations. Of the plants with haploid life cycles, most algae lack a dikaryotic phase, while most fungi have a dikaryotic phase. There are also other algae and fungi that are characterized by diploid life cycles. Lastly, plants with a haploid-diploid life history undergo an alternation of generations, either similar or dissimilar. In all of these life cycles, asexual reproduction may occur, but it is sexual reproduction that is responsible for genetic diversity. Due to variations arising separately and at different rates, the evolution of land plants did not follow a linear sequence. Before land plants, alga with mostly haploid life cycles existed, but land plants later originated from a haploid-diploid ancestor.
Meiosis and Syngamy |
| Evolution of Life Cycles |
| Haploid Life Cycle | Diploid Life Cycle | Haploid-Diploid Life Cycle |
| Ecology of Reproduction | Asexual Reproduction | Summary |
| Sources | G lossary |