The typically peltate trichomes of Pitcairnioideae feature one or four-celled, small central discs that anchor expansive shields (Figs. 2.5, 2.8D). Stalks tend to be narrow, few-celled, and equipped with less-developed protoplasts than those featured by Tillandsioideae. Shield outline ranges from more or less circular to oblong with entire to deeply incised margins. Stellate form marks the exceptional species (e.g., Fosterella; Fig. 2.5F) as do uniseriate trichomes or none at all (e.g., certain Navia). Concentricity suggestive of Tillandsioideae distinguishes occasional Pitcairnioideae, most notably members of Brocchinia, as described below (Fig. 2.5B,H,I). Navia glandulosa stands out as one of the two bromeliads reported to bear capitate, glandular hairs, in this instance predominantly on the oral bracts (Fig. 2.5K). Ronnbergia petersi (Bromelioideae) produces similar appendages combined with more typical scales on its sepals (Gross 1991).
Trichome structure identi es Pitcairnioideae allied by similar ecology, but not as faithfully as in Tillandsioideae. Narrow, radially elongated cells often produce a characteristic shield margin among the dry-growers (e.g., Hechtia) and fewer of the more mesic taxa (e.g., some Fosterella, Pepinia). Alpine Puya feature dense layers of sometimes brownish woolly scales (Fig. 7.2). Shields never develop the structure that mediates the valve-like (hydro-rectifying) action characteristic of dry-growing (Type Five) Tillandsioideae. Arid-land Pitcairnioideae usually bear trichomes with relatively opaque shields compared with relatives from moister, darker habitats.
The pitcairnioid indumentum tends to occur unevenly across the leaf surface, often more densely on the abaxial than on the adaxial side (e.g., Pitcairnia). Scales of many of the dry-growing types anchor between the costa, especially on the abaxial epidermis, where they insulate the stomata (e.g., Dyckia). Relatives from humid sites and those few species that shed foliage preparatory to drought produce scattered trichomes over one or both surfaces. Quite a few of the mesophytes (e.g., Lindmania, Navia) lack adaxial trichomes. Banded indumenta responsible for the ornamented foliage of certain Bromelioideae (e.g., Aechmea chantinii, Billbergia zebrina and especially certain Cryptanthus; Figs. 2.14F, 2.18C) and Tillandsioideae (e.g., Tillandsia flexuosa, T. hildae; Fig. 2.7I) have no parallel in Pitcairnioideae.
Special mention is due Brocchinia, which, although assigned by Smith and Downs (1974) to Pitcairnioideae, exhibit trichomes that by structure and function (Figs. 2.5B,H,I, 5.2F,G) more closely parallel counterparts in Tillandsioideae. Circular to oval shields contain different numbers of cells, but concentric alignment is common. Relatively elongate components form a rudimentary (e.g., B. reducta) to a better-de ned (e.g., B. acuminata, B. micrantha) wing peripheral to a less organized group of central cells.
Stalks may be uniseriate (e.g., B. acuminata; Fig. 5.2G) or expanded to several cells near the top (e.g., B. reducta; Fig. 2.5A). Well-developed phy-totelm architecture and demonstrated absorptive capacity among some members of this enigmatic genus suggest circumstances that also fostered the assumption of many root functions by leaves in Tillandsioideae. A hypothesis offered in Chapter 9 describes how absorptive capacity may have evolved in the trichome of Brocchinia and perhaps elsewhere among primitive Bromeliaceae.
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