The surface of a peatland lies on a column of water contained within the peat column. The peatland surface consists of a nearly complete cover of mosses (either peat mosses (Sphagnum) or true mosses (brown mosses)) that are continually pushed upward by the accumulating peat. This upward growth is limited only by the abilities of the peat and living moss layer to maintain a continuous water column that allows the living moss layer to grow. The vascular plants that grow in this water-soaked peat column produce roots that are largely contained in the small upper aerobic part ofthe peat. The mosses, however, alive and growing only from their uppermost stem apices, must maintain contact with the water column; thus, wicking and retaining of water above the saturated water column is paramount for maintenance of the moss layer. Peatland mosses have special modifications that help in this regard. Although some brown mosses have adaptations for water retention, such as the development of a tomentum of rhizoids along the stems, numerous branches along the stem that provide small spaces for capillarity, and leaves that have enlarged bases that retain water, it is in species of Sphagnum where water retention (up to 20 times dry plant weight) is greatly enhanced through a number of morphological modifications. Sphagnum has unistratose (one-celled thick) leaves consisting of alternating, large, dead, hyaline cells and small, partially enclosed, living, green cells. The walls of the hyaline cells are perforated with pores and are strengthened by the presence of cross-fibrils. Stems and branches are often encased in an outer layer of one or more rows of dead, enlarged cells. All of these hyaline cells have lost their living cell contents very early on in development and as a result the ratio of carbon to nitrogen is high. In addition to the features that allow the plants to hold water internally, the entire Sphagnum plant is a series of tiny spaces that serve as reservoirs for capillarity. The branches are surrounded by numerous, overlapping, very concave branch leaves (one-cell thick). The branches are attached to the stem in fascicles of three to five branches, half of which hang along the stem and half extend outward at more or less 90°. The fascicles of branches originate at the stem apex, and slowly develop while still close together at the apex of the stem. This group of maturing branches, the capitulum, along with the top 1-5 cm of mature stem and associated branches form a dense canopy. In total, this canopy (Figure 11) consists of numerous small spaces of different sizes and, along with the dead hyaline cells of the leaves and branches, provides the mechanism for wicking and retention of capillary water far above the actual water table, which in turn provides the framework for the aerobic peat column that is so characteristic of bogs.
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