"Algae" is an older term used for simple, aquatic plant-like organisms that lack any sort of vascular system and are generally characterized by unicellular sexual structures. Of the various types of algae, the Chlorophyta (green algae) represent the group that served as the source for the evolution of higher land plants. This evolutionary linkage is demonstrated by two fundamental similarities:

• The fact that all the details in the complex process of photosynthesis are identical with respect to the green algae and the higher land plants. Other algal groups differ respect to the detailed types of chlorophyll and accessory pigments, the details of carbon fixation and biosynthesis, and the end-products of the process.
• The strong parallels between algal life cycles and the complex, derived life cycles of higher plants.

Chlamydomonas is a single-celled green alga found in freshwater habitats such as ponds and lakes. The cells swim using a pair of whip-like flagella. As long as growth conditions are good, these organisms divide asexually (binary fission) with the best-adapted genotypes making up most of the population. When conditions begin to deteriorate (the onset of winter, small ponds drying up, etc.) the individual cells each begin functioning as isogamous gametes, fusing to form motile, diploid zygotes. The zygote almost immediately loses its four flagella and develops a very thick cell wall, forming a resting spore. This spore persists through the unfavorable period. When conditions begin to improve, each diploid resting spores divides by meiosis, producing four motile cells known as zoospores. These cells proliferate via asexual reproduction during the growing season.

Such a basic life cycle provides a way to preserve genetic diversity during unfavorable growth periods and then flood the environment with genetically diverse zoospores at the beginning of each growing season.

In the case of more complex freshwater algae, there is a multicellular haploid stage, typically consisting of long filaments of cells which are attached at one end to the bottom or other substrate (benthic). This stage, which dominates the life cycle, is developed from the haploid zoospores produced by meiosis of the diploid resting spore. Individual cells of the filament will produce unicellular, chlamydomonas-like gametes when environmental conditions deteriorate.

• Complex plants always have a multicellular haploid (gametophyte) stage in their life cycle!

In essence, the complex diploid (sporophyte) stage of the Ulva life cycle is simple a diploid version of the haploid growth form. Individual cells of the diploid plant undergo meiosis to produce the zoospores that develop into the haploid stage of the life cycle.