The taxonomic significance of trichome type and distribution in Melolobium (Fabaceae)

All species of Melolobium Eckl. & Zeyh. were examined for epidermal features and particularly the glands which are a distinctive feature amongst the southern African Genisteae. For comparative purposes, three species of Argxrolobium Eckl. & Zeyh.. all li\e species of Dichilus DC. and five species of Polliillia C.H.Stirt. were also examined for trichome type and distribution. Three trichome types are recognized in Melolobium. Trichome type and distribution provide an important insight into taxonomic relations at species level in Melolobium and sometimes even allow a distinction between regional forms. The distribution of glands (sessile and stalked) is of considerable diagnostic value in identifying species of Melolobium. A key to all the species of the genus based mainly on type and distribution of trichomes. is presented.


INTRODUCTION
Melolobium Eckl. & Zeyh. is a papilionoid legume genus restricted to southern Africa. Although about 20 species have been described, we recognize only 15 of them (a complete synonymy will be published else where). The genus consists of small shrubs or perennial herbs, characterized by their usually spiny habit, auriculate stipules and bilabiate calyces. Some species have glandular trichomes, referred to as glandular papillae bv Gibbs (1967), glandular tubercles by Polhill (1976) anil stipitate glands by Harvey (1862). Glands are also char acteristic of the Mediterranean genus Adenocarpus DC. Melolobium and related genera were originally placed in the tribe Genisteae (Harvey 1862). then transferred to Crotalarieae (Bentham 1865: Polhill 1976. 1981. and finally moved back to Genisteae by Van Wyk & Schutte ( 1995). where they are now firmly placed.
In the latest available revision of the genus, Harvey (1862) used hairs and glands as diagnostic characters, but the full extent of the variation, especially at micro scopic level, has not yet been studied. The aims of this study were: to determine the taxonomic potential of epi dermal features in Melolobium (at both species and generic levels): to record the microscopic structure of hairs and glands in this genus: and to determine the homology of glands in Melolobium and Adenocarpus.

MATERIALS AND METHODS
Hair type and distribution were investigated in all 15 of the species of Melolobium that we recognize, as well as in three species of Argxrolobium Eckl. & Zeyh., all five species of Dichilus DC. and five of the seven species of Pol hi 11 ia C.H.Stirt. A list of voucher specimens o f all species of Melolobium and the related Af rican genistoid genera used in this study is given in Table I. For light microscope studies, material taken from formalin: acetic acid: alcohol (FAA) and herbarium specimens was embedded in glvcol methacrylate (GMA) according to a modification of the method of Feder & O 'Brien (1968). This modification involves infiltrating the material for a minimum of 24 hours betw een the first two changes and for a longer period (usually at least five days) before placing in the gelatine capsules, which are then heated in the oven at 6 0' C for 24 hours to polymerize. A Porter Blum MT-1 ultramicrotome was used for sectioning and the sections were stained according to the periodic acid-Schiff/Toluidine Blue (PAS/TB) staining method. For epidermal peels, pieces of leaves w ere treated accordins to the method of Ram & Navar (1974). To study tri chome distribution, several specimens of each taxon were examined w ith a stereomicroscope. For SEM stud ies of trichomes. herbarium or washed, air-dried FAA material was used and at least two specimens of each taxon were examined using a JEOL JSM 5600 scanning electron microscope.

Trichome type
Trichome type and distribution in Melolobium and related African genera are summarized in Table 2. Three trichome types were recognized in Melolobium: uniseri ate hairs with a long narrow terminal cell and two or three short basal cells ( Figure 1A In the subfamily Papilionoideae, uniseriate hairs con sist ol three cells: a frequently enlarged epidermal cell, serving as a basal cell: a short stalk cell, which occa sionally has special contents and is suberized: and an elongated terminal cell (Solereder 1908). In Melolobium.  however, some hairs have one basal cell, whereas others have two. In the latter case, the basal cell appears to have divided periclinally ( Figure IB). The basal cells are structurally similar to other epidermal cells and the stalk cell is very thick-walled ( Figure IB). In Argyrolobium, Dichilus and Polhillia on the other hand, there is always one basal cell, which like the other epidermal cells, is papillate ( Figure 1C). In this case the stalk cell is also markedly thickened. According to Solereder (1908) the terminal cell is often uniformly or spirally thickened, with verrucose or peg-shaped irregularities on the sur face. Scanning electron microscopy shows that in the examined species there are two types of surface sculp turing of the terminal cell: striated and verrucose. In all species of Melolobium. hairs have striated surfaces ( Figure 4A). whereas in all species of Dichilus they are verrucose ( Figure 4B). In Argyrolobium and Polhillia. both striated and verrucose hairs are present (Schutte 1988).
Stalked glands and sessile glands occur only in Melolobium species and not in any of the other genera.
Structural details o f these two types of glands are record ed and illustrated for the First time. Stalked glands have a spherical, unicellular head and a multicellular stalk consisting of several elongated cells (Figure 2A-D).
According to Solereder (1908), the glands in Melolobium are unicellular and consist of a short globular head. Polhill (1976) likened them to those found in Adenocarpus, but Solereder (1908) described the glands of Adenocarpus as 'm ulticellular glandular shaggy hairs, columnar in shape and broadened in a capitate manner at their apex'. Gibbs (1967) referred to Adenocarpus glands as glandular papillae arising 'as outgrowths of columnar shaped epidermal cells'. Examination of these glands, however, shows that they are neither shaggy hairs nor papillae, but rather multicellular glands ( Figure 2F

+ V s G s + + + + + + V S s G s + V s s G s + V s s G s + V G G G G + + + + + + V G G G G R + R -R + V " ■ S ■ ■ * ---R V
_ _ _    The co-occurrence of structures is generally indica tive of non-homology. Since stalked and sessile glands never co-occur in Melolobium, a sessile gland may be a stalked gland in which development was merely arrested at the unicellular stage. Sessile and stalked glands have diagnostically different distributions at the species level with no variation at all within species. Hairs, on the other hand, are more variable in distribution and can be used to distinguish between different populations within some of the species.

+ + + + + + St ----+ + + + + + V + V ----+ + + + + + St
The function of glandular trichomes in Melolobium is not yet clear, but many species are viscid. Examination of epidermal peels of the leaves revealed the heads of the glands to have dense protoplasts (Figures 2A, B, D; 3), further suggesting that these structures might be secre tory in nature. In Adenocarpus, kthe inner cells of the glands break down at maturity to produce a viscous secretion' (Gibbs 1967). Glandular trichomes are known to secrete a large number of different substances, includ ing water, salt, nectar, mucilage, terpenes and digestive enzymes (Esau 1977). Studies are being carried out to determine the chemical nature of the contents of these glands.

Hair distribution
In Melolobium the distribution of hairs on the leaflet blade varies greatly, even within a species, but is highly consistent within various forms or provenances. where they are absent. In general, hairs are less frequent on the adaxial surface than on the abaxial one. Hairs can be used to distinguish many of the species of Melolobium. M. adenodes is allied to M. humile (both glandular), for example, but the former is subglabrous and the latter is densely hairy. M. aethiopicum (L.) Druce superficially resembles M. alpinum, but leaves of the for mer are hairy on both surfaces, whereas the latter is only sparsely hairy on the upper surface. In all species of Melolobium, glands (whether stalked or sessile) and hairs co-occur. The distribution of hairs is not correlated with the type of glands.

CONCLUSIONS
The type and distribution of glands is of diagnostic significance at the generic level in African Genisteae and at the species level within Melolobium. The type and dis tribution of hairs is of taxonomic value at both species and population (provenance) levels. Within the Genis teae, the microscopic structure of hairs and glands is unique in Melolobium. The distinctly sp in y ...................................