Gnarled Trichomes: An Understudied Character in Pteridium

Abstract

-One of three characteristics used by Tryon to distinguish southern bracken ferns (Pteridium) of subsp. caudatum from northern taxa assigned to subsp. aquilinum is the farinaceous appearance of the intervein areas on the lower surface of the ultimate laminal segments. This feature is due to minute, generally shortened, thickened, bent and tuberculate hairs, which we describe here as gnarled trichomes. Gnarled trichomes are consistently present in esculentum, even in plants with glabrous lateral veins, but are reduced in density in shade form fronds. Neither P revolutum nor P yarrabense (probably = P esculentum x P revolutum) has gnarled trichomes. South and Central American specimens of P aquilinum vars. arachnoideum and caudatum show either normally tapered or gnarled intervein trichomes, or entirely lack intervein hairs. If present, gnarled trichomes are generally short and strongly tuberculated in var. arachnoideum, but often longer with multiple bends and a hooked tip in var. caudatum. Development of the lower laminal indumentum in appears to involve three distinct but interacting morphogenetic fields comprising respectively the midvein surfaces, lateral vein surfaces and intervein areas. Further investigation is required to clarify whether vars. arachnoideum and caudatum are both polymorphic for presence versus absence and normally tapered versus gnarled intervein trichomes, or whether these taxa and their possible hybrids should be redefined. With masterly understatement, Page (1976) has written that, Pteridium has always proved a somewhat difficult genus within which to produce a satisfactory taxonomic treatment. Factors contributing to this difficulty include: i) the limited number of taxonomically useful characters applicable between the population and specific levels in the bracken ferns; ii) the frequent occurrence of intermediates between named forms (Tryon, 1941); and iii) the extraordinary phenotypic plasticity exhibited by the brackens in relation to local environmental conditions (Page, 1976). No world-scale taxonomic overview of has been undertaken since the work of Tryon (1941), reviewed by Page (1976). Recent studies of the brackens of Australia (Brownsey, 1989), Mexico (Mickel and Beitel, 1988), Venezuela (Ortega, 1990), and Europe (Page and Mill, 1995) have highlighted the need for such a global revision. It therefore seems appropriate to reevaluate those characters that have previously proven of diagnostic value in Pteridium, but which remain poorly understood, with the goal of achieving improved resolution of relationships. One such characteristic is whether the lower epidermis of the frond lamina between the veins is, or is not, farinaceous in appearance. Tryon (1941, p. 8) states of Pteridium: Some varieties have a farinaceous appearance on the lower surface of the ultimate segments. This is a minute epidermal outgrowth, occurring between the veins, which under high magnification resembles a sparsely branched coral fungus. It may be an excretion but is not affected by alcohol or dilute acid. Tryon's (1941, Fig. 13, Plate 650) This content downloaded from 207.46.13.58 on Wed, 12 Oct 2016 04:39:19 UTC All use subject to http://about.jstor.org/terms THOMSON & MARTIN: GNARLED TRICHOMES IN PTERIDIUM TABLE 1. Nomenclature for taxa of discussed in this paper. Names of subspecies and varieties according to Tryon (1941), as modified by Page (1976), are shown alongside alternatives used where the taxa are recognised as species or hybrids (see also Brownsey, 1989; Mickel and Beitel, 1988; Ortega,