Pit membrane types vary within families but appear relatively conserved within genera. While some taxa display torus-margo bordered pits (60%), many others do not contain tori (40%), and at least some taxa without tori occur within each of the four extant phyla (Coniferophyta, Cycadophyta, Ginkgophyta, and Gnetophyta). Gymnosperm taxa vary greatly in their inter-conduit pit structure, with different types of pit membranes and pitting arrangements. Within the xylem of gymnosperms, axial transport occurs predominantly via tracheids, although 10–40% of gymnosperm taxa, particularly within the Gnetophyta and Cycadophyta, also contain vessels. Although less appreciated, xylem conduits are also diverse in structure among extant gymnosperm taxa. Gymnosperm taxa are incredibly diverse in many traits, including taxa with evergreen or deciduous leaves broad, compound, needle-like or scale-like leaves trees, shrubs, and lianas and taxa with seed cones that range from fleshy to woody. The most common pit membrane type and pitting arrangement is exemplified by Pinus xylem and is represented by tracheids with mostly uniseriate pitting, some biseriate pitting within the earlywood (not shown), and Type A pit membranes. Additionally, focusing on torus aspiration as a key trait likely also results in a failure to fully appreciate other potential pit membrane morphologies, such as in species that have torus extensions, which are thickened segments of the pit membrane that connect the torus to the pit chamber wall and that are found in Abies (Sano et al., 1999 Dute et al., 2008), Tsuga (Liese, 1965), and Widdringtonia (Pittermann et al., 2010) or in species with vestures, such as found in some Gnetum, although thus far tori and vestures have not been described as co-occurring within a single Gnetum pit membrane. For instance, many studies have focused on torus overlap and pit aspiration (Delzon et al., 2010 Pittermann et al., 2010), but in species without tori, such as Thuja occidentalis, pit membranes do not aspirate (Liese and Bauch, 1967).
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