Haplontic or Zygotic Life Cycle
This cycle is characterized by a single predominant haploid vegetative phase, with the meiosis taking place upon germination of the zygote. Chlamydomonas (Chlorophyta) (Figure 1.19) exhibits this type of life cycle.

FIGURE 1.19 Life cycle of Chlamydomonas sp.: 1, mature cell; 2, cell producing zoospores; 2', cell producing gametes (strain+ and strain-); 3, zoospores; 3', gametes; 4', fertilization; 5', zygote; 6', release of daughter cells. R!, meiosis; a.r., asexual reproduction; s.r., sexual reproduction.

Diplontic or Gametic Life Cycle
This cycle has a single predominant vegetative diploid phase, and the meiosis gives rise to haploid gametes. Diatoms (Figure 1.20) and Fucus (Heterokontophyta) (Figure 1.21) have a diplontic cycle.

FIGURE 1.20 Life cycle of a diatom: 1, vegetative cell; 2, 3, vegetative cell division; 4, minimum cell size; 5, gametogenesis; 6, 7, fertilization; 8, auxospores; 9, initial cells. R!, meiosis.

FIGURE 1.21 Life cycle of Fucus sp.: 1, sporophyte; 2, anteridium; 2', oogonium; 3, sperm; 3', egg; 4, zygote; 5, young sporophyte. R!, meiosis.

Diplohaplontic or Sporic Life Cycles
These cycles present an alternation of generation between two different phases consisting in a haploid gametophyte and a diploid sporophyte. The gametophyte produces gametes by mitosis; the sporophyte produces spores through meiosis. Alternation of generation in the algae can be isomorphic, in which the two phases are morphologically identical as in Ulva (Chlorophyta) (Figure 1.22) or heteromorphic, with the predominance of the sporophyte as in Laminaria (Heterokontophyta) (Figure 1.23) or with the predominance of the gametophyte as in Porphyra (Rhodophyta) (Figure 1.24).

FIGURE 1.22 Life cycle of Ulva sp.: 1, sporophyte; 2, male zoospore; 2', female zoospore; 3, young male gametophyte; 3', young female gametophyte; 4, male gametophyte; 4', female gametophyte; 5, male gamete; 5', female gamete; 6–8, syngamy; 9, young sporophyte. R!, meiosis.		FIGURE 1.23 Life cycle of Laminaria sp.: 1, sporophyte; 2, male zoospore; 2', female zoospore; 3, male gametophyte; 3', female gametophyte; 4, sperm; 4', egg and fertilization; 5, zygote; 6, young sporophyte. R!, meiosis.
FIGURE 1.24 Life cycle of Porphyra sp.: 1, male gametophyte; 10, female gametophyte; 2, sperm; 2', egg; 3, fertilization and zygote; 4, spores; 5, sporophyte; 6, male spore; 6', female spores; 7, young male gametophyte; young female gametophyte. R!, meiosis.