Is there a tapetum in the hornwort capsule? Evidence from the sporogenesis of Phaeoceros

Abstract

The tapetum is an important tissue in the sporangia of tracheophytes that provides nutrition for developing spores and participates in sporoderm construction, but the data on the tapetum in the sporangia of bryophytes and, in particular, hornworts, are contradictory. Some investigators considered the capsule wall and pseudoelates a tapetum in hornworts, but others suppose that the tapetum is absent in hornwort capsules at all. The structure of the sporocytes and spores, pseudoelaters, the lining layer of the capsule wall, and the outer layer of the columella of Phaeoceros laevis were studied at successive developmental stages by light, fluorescence, and transmission electron microscopy. We revealed that sporocytes accumulate carbohydrates that are required for the construction of the sporoderm and the storage of nutrients in mature spores before the formation of a special wall. Such a feature significantly distinguishes hornworts from mosses and angiosperms. Pseudoelaterocytes and pseudoelaters do not possess any ultrastructural features, indicating that they provide the sporocytes and spores with nutrition. Cells of the lining layer of the capsule wall and the outer layer of the columella do not possess characteristic ultrastructural features of secretory tissue at any developmental stage (in contrast to the moss tapetum), but they apparently participate in the transfer of nutrients in the capsule locule from the foot at the time the sporocytes are actively growing and accumulating nutrients. We accept the strict interpretation of the tapetum as a tissue that lines the capsule locule and has the specialized ultrastructure characteristic of the secretory tissues. Therefore, contrary to the opinion of some researchers, we believe that neither pseudoelaters nor the lining layer of the capsule wall should be considered a tapetum. Columella cells have a structural specialization for upward transport of assimilates, and they should not be regarded as a tapetum. We suppose that tapetum was absent in the common ancestor of the hornworts as well as in the common ancestor of embryophytes.