Adaptive Modes of the Moss Sporophyte

The function of the sporophyte is to produce-via meiosis-spores, and to disperse these sexually reproduced diaspores as efficiently as possible; thus, selection pressures that serve to maintain and modify this function are critical in understanding the evolution of the sporophyte generation. These pressures are significantly different in hygrophytic, mesophytic, and xerophytic habitats, and each of these habitats must be examined separately to ascertain the selection gradients and the structural modifications of plants living in them. Correlations between particular structural features and specialized habitats can be observed, and inferences of function and adaption can be drawn from these data. Reduction in many sporophyte characteristics can be correlated with xerophytic habitats. In particular; seta length is often shorter, capsules broader and erect, and the peri- stome reduced or absent. Peristome reduction can take place by at least two processes-(l) fusion of parts and (2) reduction of parts. Xerophytic habitats are colonized by mosses that are either tolerant of desiccation or by those that can avoid desiccation periods. The latter have a shortened life cycle and many adap- tations different than those that are more poikilohydric. Although the prev- alent trend in moss sporophytic evolution is one of reduction, amplification of the sporophyte can be found in mesophytic taxa and, in particular, in the entomoph- ilous members of the Splachnaceae. An understanding of sporophyte modifications in relation to habitat preferences will greatly clarify ideas of evolutionary paral- lelisms and adaptive specialization. The Bryophyta can be best defined as those archegoniate plants that have the deter- minate sporophyte generation attached to and partially dependent on the gametophyte generation for the duration of the life cycle. Opposed to this, spore-bearing tracheophytes have the largely indeterminate sporophyte generation growing independent from the ga- metophyte (or in seed plants the gametophyte is surrounded by sporophyte tissue and dependent on the latter). These two situations do not necessarily imply two monophyletic groups, rather they indicate two separate life styles initiated by selection pressures on early land plants. Bryophytes, with the young sporophyte protected in the gametophyte, need not undergo a second external germination event; however, the young sporophyte of spore-bearing tracheophytes must establish itself separately from the gametophyte. The partial dependency of the sporophyte upon the gametophyte is, in the long term, a great disadvantage, since the bryophyte is faced with the evolutionary enigma of very different gametophytic and sporophytic selection pressures that must be combined into a single plant. In tracheophytes, the selection pressures inherent in a gametophyte can unilaterally be acted upon evolutionarily, separate from the very different selection pressures acting upon the sporophyte. The primary function of the sporophyte is to produce-via meiosis-spores, and to disperse these sexually reproduced diaspores as efficiently as possible. Thus, even though