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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.
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.
Coleochaete, Chaetosphaeridium, stoneworts, and embryophytes all produce a multicellular haploid body, and all produce large non-motile 'female' gametes and small motile 'male' gametes.
Coleochaete and embryophytes retain their zygotes on the maternal gametophyte, whereas Chaetosphaeridium expels its ova before fertilization.
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.
Two somewhat conflicting theories (the homologous theory and antithetic theory) of the origin of alternating generations, specifically the origin of the sporophyte, in embryophytes (land plants) have had respective supporters for approximately a century.
In any case, overwhelmingly, the sporophytes of embryophytes do not closely resemble the gametophytes on which they depend.
The larger flagellar root is a "band" with many microtubules (perhaps 60 or so) and is associated (toward the base) with a distinctive multilayered structure (MLS) composed of microtubules and laminate plates; this composite structure is similar to that found in sperm of embryophytes (cf.
lutein, beta-carotene) with embryophytes and, associatedly, similar chloroplast structure and thylakoid arrangement (Van den Hock et al.
Regardless, the Viridiplantae are considered by virtually all authors to represent generally related groups of organisms, some details not withstanding; in most cladistic analyses it seems clear that, among algae, charophytes place the closet to lower embryophytes.
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