Phylogenetic Transitions in the Chloroplasts of the Anthocerotales I. The Number and Ultrastructure of the Mature Plastids

Representatives of three genera of anthooerotes were examined: Phaeoceros, Notothylas, and Megaceros. Species of the first two genera were found to exemplify the typical anthocerote plastid condition. This condition is characterized by the presence in each cell of the gametophyte of only a single large chloroplast containing a “multiple” pyrenoid. The genus Megaceros, however, proved to be quite different. In two species of Megaceros the pyrenoid was observed to be composed of a highly subdivided thylakoid system of even greater complexity than the “multiple” pyrenoids of Phaeoceros. In another species only an indistinct “pyrenoid‐like” area was noted while in a fourth species no evidence was found for any internal differentiation. Associated with these changes in plastid structure there are corresponding alterations in the number and the size of the chloroplasts. Together they indicate an evolutionary trend away from a primitive, algal‐like condition to a more advanced land plant form.