Bromeliaceae, with far fewer species and almost exclusively neotropical distribution, nevertheless rival Orchidaceae for variety of epiphytic mechanisms (Table 8.7) and vastly exceed the latter's biomass in tropical American forests. Tank habits have evolved independently in two brome-liad subfamilies, and in all three if sometimes-epiphytic Brocchinia (Fig. 4.25E) is correctly assigned to Pitcairnioideae (Benzing et al. 1985). A rosu-late shoot was required for each transference of absorptive role from root to foliage. Ancestry was apparently mesic in both Tillandsioideae and Pitcairnioideae (Fig. 2.4; Benzing and Renfrow 1971a; Medina 1974; Benzing et al. 1985 - but see Pittendrigh 1948); tank shoots show C3 photosynthesis in each subfamily. Bromelioideae, with about 500 species capable of creating substitutes for ground soil in leaf bases, are fundamentally CAM plants that probably acquired nocturnal C02 fixation and impoundment as terrestrials in arid habitats. Specialization for PS epiphytism - in effect for greater stress tolerance - has proceeded farthest in the derived atmospheric forms of Tillandsioideae (Benzing et al. 1985), where absorbing trichomes (Fig. 3.21) are perfected to the highest degree while the vegetative apparatus is reduced to simplest form (Fig. 4.25A-D).
Early Bromeliaceae were clearly predisposed to epiphytism by the presence of a suitable epidermal appendage and habit. Here, life in the canopy is based on a modified shoot with the foliar trichome as its keystone feature (Table 8.7). Leaf bases need not be highly specialized to tap tanks continuously filled with moist humus and the organisms required to reduce it to plant nutrients, but atmospheric epiphytism requires an extraordinary foliar indumentum (Benzing and Pridgeon 1983). Myrmecophytism, and a single case of epiphytic carnivory (Catopsis berteroniana; Givnish et al. 1984), are also associated with modified shoot surfaces and tubular rosettes (Benzing 1970b). Hypotheses concerning how the bromeliad foliar epidermis may have acquired its current function and importance are described elsewhere
(Pittendrigh 1948; Benzing et al. 1985). (Briefly: Contrary to Pittendrigh's proposition that absorptive function would emerge only under drought selection, Benzing et al. posited a mesic, infertile, ancestral habitat and a foliar epidermis and habit that assumed their unusual characteristics to promote utilization of impounded humus or perhaps animal prey.) Bromeliad seeds are disseminated by birds (Bromelioideae; Fig. 5.2G) or wind (Pit-cairnioideae and Tillandsioideae; Fig. 5.2K). Pollination syndromes are diverse.
Aroid, by comparison with bromeliad or orchid, epiphytism is neither as advanced nor as versatile, although a capacity for life in tree crowns has originated at least three times in the family (Madison 1977). There are no reports of CAM here (Table 2.1) although perhaps CAM-cycling exists (Ting et al. 1985b), and overlapping foliage that might mitigate drought lacks the watertight quality of inflated bromeliad leaf bases. Trash-basket catchments (Fig. 1.5) sometimes trap falling litter but little moisture. Roots fail to produce velamina as elaborate as those of the most drought-tolerant Orchida-ceae, nor is there any indication that these organs can contribute substantially to the plant's carbon budget as can those of some orchids. Seasonally deciduous leaves on green or tuberous stems occur in Philodendron and Remusatia, respectively, but these are minor themes represented by few species. Arboreal existence in Araceae is based predominantly on two mechanisms, both humus-based (Table 8.7): impoundment, seen in short-stemmed Anthurium and some Philodendron; and secondary hemiepiphyt-ism (Fig. 1.9), a more widespread phenomenon often encountered in Anthurium, Philodendron, and Rhaphidophora. Velamentous roots, and vining habits truncated by progressive stem decay are responsible for aroid hemiepiphytism. Both sympodial (e.g., Philodendron) and monopodial (e.g., Pothos) habits are involved. Water and nutrient balance exhibit no obvious modifications for arboreal life, but they have not been examined closely. Ant nest-gardens are utilized by some Anthurium and Philodendron. Baccate fruit is an integral part of the aroid epiphytic syndrome, but it occurs throughout the family without habitat restriction. Pollinators range from beetles to euglossines. Specialized pollen vectors, including coleopterans that appear to be unexpectedly faithful to specific flowers (C. Dodson pers. comm.), may have prompted radiation of Anthurium and possibly other genera.
Cyclanthaceae, the only other nonorchid monocot family with a sizable epiphyte contingent (largely Asplundia; Table 1.1), mostly utilize the forest canopy as permanently ground-rooted climbers and secondary hemiepi-
phytes. True epiphytism occurs in Sphaeradenia and Stelestylis (G. Wilder pers. comm.). Stems and internodes are shorter than those of related hemiepiphytes.
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