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All land plants (embryophytes), including the bryophytes, have a well-defined alternation of two generations with meiosis occurring within distinctive sporangia of the sporophyte.
As an early diverging lineage of embryophytes, the liverwort clade is as old or older than any other land plant lineage (Mishler & Churchill, 1984; Hedderson et al., 1996, 1998; Garbary & Renzaglia, 1998; Qiu et al., 1998; Renzaglia et al., 2000; Von Konrat et al., 2010a; b).
Streptophyte algae and the origin of embryophytes. Annals of Botany 103: 999-1004.
Despite these advances, there are still many uncertainties about how embryophyte life cycles were derived from a charophycean life cycle.
In this section, I will briefly summarize the limited data available about the life cycles of Coleochaete, Chaetosphaeridium, and stoneworts (Charales), and then relate this information to the evolution of embryophyte life cycles.
Modern defenders of the homologous theory (e.g., Remy, 1980; Remy & Hass, 1991) argue that post-zygotic mitosis first evolved in an embryophyte ancestor that dispersed its zygote.
I will not enter into the details of these arguments because most of these taxa are now known to be distant relatives of embryophytes. However, I will discuss various interpretations of the life cycle of Coleochaete, an alga whose life cycle had been described by Pringsheim (1860).
Rather, it is charophytes (e.g., Coleochaete, Chara, Nitella) that are closely related to land-plant (embryophyte) ancestors (cf.
But if we allow that the land-plant (embryophyte) lineage is traceable to charophycean forms (i.e., to these sorts of algae), does this help us decide which theory of alternating generations, and sporophyte origin, to adopt?
Coleochaete indeed appears to offer developmental clues to embryophyte origins (Graham, 1984).
The strongest evidence at present (and the beginnings of resolution of the question of the initiation of alternating generations in land plants) comes to light when we understand which algal group, generally, is most probable as representative of forms involved in embryophyte origin.
Interpretation morphologique de l'embryon ches les Embryophytes, avec application au cas des Graminees (Poaceae).
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