Nutrient Relations: Translocation and Transport

Figure 1. Bazzania trilobata and other bryophytes growing together. Note the dead tissue in the lower right part of the clump, where nutrient sinks can be mobilized to supply growing apical tissues. Such clumps can play a significant role in the distribution of forest nutrients. Photo by Michael Lüth. One of the most poorly understood abilities of bryophytes by "vascular" botanists is the ability of bryophytes to transport nutrients within the plant. Making the assumption that transport of nutrients occurs through vascular tissue (xylem and phloem), they have assumed that the "non-vascular" bryophytes are unable to move substances from one part of the plant to another. Hence, the assumption has been that as bryophytes die (Figure 1), decomposition will return the component nutrients. But while bryophytes lack tracheids, vessels, and sieve cells, they do not lack the ability to transport substances from one part of the plant to another, i.e. translocation. In some cases, such as Polytrichum, they actually transport substances through their leptoids (phloem-like cells; Figure 2) and hydroids (xylem-like cells; Figure 2). Those mosses such as Polytrichum with well-developed leptoids form a leptome, similar to the cylinder of phloem in a tree trunk. The collective hydroids in the center of the stem form the hydrome, also known as the hydram. But it is clear that lack of even these special conducting cells is no deterrent to transport or to translocation in bryophytes. Hence, we can find nutrient elements in a number of locations within and upon the plant (Table 1). One aid to the transport of substances from cell to cell is the presence of plasmodesmata in the cell walls (Mahmoud 1965; Oliver & Bewley 1984). These connecting threads permit substances to move from cell to cell without traversing cell membranes, although the movement is undoubtedly slower than that of the movement of water in the interstitial capillary spaces of cell walls.