Floral Mechanics in Phlomis (Lamiaceae)

Abstract

Closed corollas that have to be opened by pollinators form a protection of pollen or nectar against adverse weather conditions or against thieves, e.g. in many mask-, gulletor flag-blossoms. Only occasionally thieves can reach the pollen (Brantjes, 1980) or the nectar (Knoll, 1922) through the entrance of such flowers. Usually, thieves perforate the corolla. The legitimate pollinator has to be able to open the flowers and, as can be concluded from the occurrence of closed flower types, mostly in plant species with mellitophilous characters, the opening capacities seem to be restricted to bees (Hymenoptera). These bees vary considerably in size and also in power. This has implications for their abilities to handle a particular flower species, e.g. to trip flag-blossoms of the Fabaceae, among which are many crop plants. We designed a means of studying the mechanical aspect of the flower-pollinator interactions and chose a flower ( Phlomis fruticosa L., Lamiaceae) pollinated by what are said to be the strongest bees: Carpenter bees ( Xylocopa spp.) (Van der Piji, 1972). Pallas (1973), working under the supervision of Van der Piji, confirmed that in Tessalie (Greece) Xylocopa violacea was the only nectar drinker and a successful pollinator of Phlomis fruticosa . In the botanical garden, Hortus de Wolf, Haren, Netherlands, where the present experiments were carried out, Carpenter bees are absent. Occasional bumblebees operate the flowers and therefore, acted as 'stand in'. Bracts surround the bases of the aggregated flowers, presumably forming a protection against puncturing nectar robbers (Van der Piji, 1972). The corolla consists of a tube, a lower lip and an upper lip. The strongly recurved lower lip has a median groove and two lateral lobes [Fig. 1(a), (b)]. The profile gives much of the strength to the lower lip, which does not deform under the pressure produced by the visitors. A broad hinge part connects the hood-like upper lip on to the corolla tube [Fig. 1(a), (b), (d), (e)]. The proximal part of the hood is compressed laterally with only its outer parts diverging [Fig. 1(b), (f)]. The sides of the strongly-inflated distal part of the hood touch each other [Fig. 1(a), (g)]. The top of this part bears a longitudinal fold [Fig. 1(b), (g)]. In the tubular corolla the stamen filaments are continuous with the corolla wall [Fig. 1(c)]. Just below the hinge the filaments separate from the corolla and arch inside the curve of the hood. Thin curled hairs interconnect the four filaments to form a stiff bundle [Fig. 1(e), (f)], which sits confined in a groove above the lateral compression [Fig. 1(e), (f)]. In the top part of the corolla tube the style lies between ridges formed by the filament bases. In the hood, the style is situated between the filaments until its proximal part extends beyond the corolla [Fig. 1(a), (b)]. In this position the stigma will touch the visitor before the anthers do, and self pollination seems to be prevented. The plant was found to be self compatible in Greece, but outcrossing will be the rule because of the