Viscin Cells in the Dwarf Mistletoe Arceuthobium ... - Davidsonia

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78 Collection and Observation of Seeds and Viscin Cells Following Explosive Discharge In the lab, at least 10 seeds per sample date were removed from the cheesecloth with forceps, wetted, placed on glass plates, and photographed with an Olympus SZH DFplan dissecting microscope (Olympus, Tokyo, Japan). Those same seeds were allowed to dry on the glass plates overnight, and were photographed again. Whole mounts of both wet and dry viscin cells were made and photographed with the Nikon Optiphot compound light microscope. Measurements of outside-fruit viscin cell lengths were obtained as before for both the wet and dried viscin cells for each of the three seed collection dates. Viability of the viscin cells was assessed using 5 seeds picked off the cheesecloth wraps on each of the three collection dates. Seeds were presoaked in distilled water for 24 hours, placed into sterile Petri plates, and submerged in 1% (w/v) 2,3,5-triphenyl tetrazolium chloride (2,3,5- TTC) in the dark for 72 hours (Scharpf and Parmeter 1962). The staining reaction of the viscin cells was examined at the light microscope level. Red staining indicated operation of oxidation-reduction biochemistry, a sign of viability. Results and Discussion The mature fruit, which was bicoloured and recurved (Figure 1), contained a single seed consisting of an embryo and endosperm enveloped in a conspicuous uniseriate layer of mucilaginous viscin cells (Figure 2). The viscin tissue surrounded three-quarters of the embryocontaining apical region of the seed, i.e., the embryo-pole. Rehydrated seeds collected from the cheesecloth 1, 3, and 5 days following explosive discharge had the same general appearance (Figure 3). Discharged seeds with the viscin tissue layer were, on average, 2.5 mm long. The viscin cells were green and contained chloroplasts that were visible under the compound microscope (Figure 4). While chloroplasts have been observed in the endosperm and embryo of Arceuthobium species (Hawksworth and Wiens 1996), this is the first report of chloroplasts in the viscin cells of any mistletoe. If they are functional, they may have a biosynthetic role during the early stages of parasite connection to the host. The cellulose fibrils of the viscin cell walls had a distinct helical arrangement (Figure 4), which unlike elaters of fern sporangia (Mauseth
Davidsonia 17:3 79 Image: Cynthia M. Ross Figure 1. The Arceuthobium americanum fruit, seed, and viscin tissue. Mature recurved fruit photographed two days prior to explosive discharge. scale bar = 4 mm. 1988), involved a spiral coiling of the cellulose fibrils of the primary cell wall along the long axis of the cell. This has not been previously described in any other plant cells and requires further study. The resemblance of these viscin cells to “springs” may reflect a function in explosive seed discharge, adhesion, or both. The elaters of fern sporangia are helical cells with a role in spore discharge but the
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Page 27 and 28: Davidsonia 17:3 97 REFERENCES Allen
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