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Telopea Vol. 6(4): 1996 
Discussion 
Systematic characters 
Secondary thickening meristem (STM) Similarity in habit of some of these genera, 
especially Xanthorrhoea and Kingia, is a result of different growth mechanisms, 
representing two non-homologous systems, either with or without a secondary 
thickening meristem (STM). Waterhouse (1987) cited this as evidence that 
Xanthorrhoeaceae sensu la to is a 'monstrously unnatural' family. Fahn (1954), 
Waterhouse (1967) and Staff & Waterhouse (1967) showed that Xanthorrhoea has an 
STM, but that this is entirely lacking in Dasypogon and Kingia, in which stem 
thickening is palm-like, with a broad apical region and sunken apical meristem (i.e. 
an extensive primary thickening meristem: PTM). Virtually all monocots have a 
PTM, but among tree-forming monocots this has developed along different lines, 
probably more than once, either as an extensive apical PTM, as in palms, or as the 
STM, at some distance from the apex. Among the herbaceous taxa of the group in 
question, Fahn (1954) could not find an STM in rhizomes of CImnaexeros, Calectasia 
and Dasypogon, but reported an STM in Loinandra. Later authors (e.g. Stevens 1978, 
Bedford et al. 1986) have tended to discount this observation, but our investigations 
confirm that Lomandra does indeed have an STM (Fig. la), in common only with 
some other woody Asparagales (Rudall, 1991, 1995). We also confirm that an STM 
occurs in Xanthorrhoea, but not in Calectasia, Dasypogon or Kingia. An STM also occurs 
in Thysanotus (Rudall 1995), which resembles Lomandra (but not Xanthorrhoea) in 
ovule and embryo sac structure (Rudall 1994). 
Leaf anatomy Taxonomically significant leaf anatomical characters in the genera 
investigated include vascular bundle orientation, and position of sclerenchyma in 
relation to bundle sheath cells and epidermis. Several of the taxa have girders 
(sclerenchyma linking the vascular region with the epidermis), but these are clearly 
non-homologous between the various groups: in Xanthorrhoea they are of mesophyll 
origin (not associated with vascular bundles), in Baxteria mesophyll or outer bundle 
sheath, and in the Lomandra-group from the inner bundle sheath. Fahn (1954) 
described modified substomatal cells in Baxteria, Kingia and sometimes in Dasypogon 
and Xanthorrhoea. Xerolirion resembles the Lomandra-group in having sclerenchyma 
girders from the inner bundle sheath. 
Although species-level differences are not emphasised here, leaf anatomy may well 
provide useful characters at this level in some genera, such as Xanthorrhoea and 
Lomandra. Indeed, Fahn (1954) produced a key to identification of Lomandra species, 
based entirely on leaf anatomical characters. Leaf anatomy may well help to elucidate 
relationships within the Lomandra-group (Acanthocarpus, Chamaexeros, Lomandra and 
Romnalda) and with putatively allied arthropodioid genera such as Arthropodiiim, 
Cordyline, Chamaescilla, Eustrcphus, Smoerbaea, Thysanotus and Trichopetalum (see below). 
Silica The presence of silica in Kingia, Baxteria, Dasypogon and Calectasia supports 
their inclusion in the commelinoid clade, in which it is commonly present (Chase et 
al. 1995b). Silica bodies do not otherwise occur in Lilianae sensu Dahlgren et al. 
(1985), except in orchids (and Hanguana, which is probably commelinoid). Silica 
bodies are also absent from basal monocot groups, such as Arales, Alismatales and 
Najadales. In orchids, silica bodies are present in the vascular bundle sheaths, not in 
the epidermis (Dahl Moller & Rasmussen 1984), and are of two types: either spherical 
(in some epiphytic orchids) or conical (in both terrestrial and epiphytic life-forms). 
Since they do not occur in all orchids, and are absent from other Lilianae, silica 
bodies probably originated de novo in Orchidales and were subsec]uently lost in at 
least one group, as Dahl Moller & Rasmussen (1984) also concluded. 
