Conducting tissues and phyletic relationships of bryophytes

Internal specialized conducting tissues, if present, are restricted to the gametophytic generation in liverworts while they may occur in both generati ons in mosses. Conducting tissues are unknown in the anthocerotes. Water-co nducting cells (WCCs) with walls perforated by plasmodesma-derived pores oc cur in the Calobryales and Pallaviciniaceae (Metzgeriales) among liverworts and in Takakia among mosses. Imperforate WCCs (hydroids) are present in br yoid mosses. A polarized cytoplasmic organization and a distinctive axial s ystem of microtubules is present in the highly specialized food-conducting cells of polytrichaceous mosses (leptoids) and in less specialized parenchy ma cells of the leafy stem and seta in other mosses including Sphagnum. A s imilar organization, suggested to reflect specialization in long-distance s ymplasmic transport of nutrients, also occurs in other parts of the plant i n mosses, including rhizoids and caulonemata, and may be observed in thallu s parenchyma cells of liverworts. Perforate WCCs in the Calobryales, Metzge riales and Takakia, and hydroids in bryoid mosses, probably evoked independ ently. Because of fundamental differences in developmental design, homology of any of these cells with tracheids is highly unlikely. Likewise, putativ e food-conducting of bryophytes present highly distinctive characteristics and cannot be considered homologous with the sieve cells of tracheophytes.