SNAP-TENTACLES AND RUNWAY LIGHTS:

SUMMARY OF COMPARATIVE EXAMINATION OF DROSERA TENTACLES

BY IRMGARD AND SIEGFRIED R.H. HARTMEYER

Keywords: Droseraceae, Drosera glanduligera, Drosera hartmeyerorum, lens-tentacles, snap-tentacles, systematics

published in Carnivorous Plant Newsletter (ICPS) Vol. 39/4:101-113

(enhanced with additional photos and video links on this website)

Introduction

        In 2003, Davion mailed his private video-shots of the phenomenon, including some seeds, asking us to confirm his observation. Equipped with camcorder and USB-microscope, we examined the minute, always hungry and absolutely nasty to cultivate sundew, during two winter seasons. By 2005, we had collected enough material to fully confirm Davion’s observations and to publish even more amazing features, such as the unique ontogenetic development (Hartmeyer & Hartmeyer 2005). We were also able to produce our Hunting Veggies^® video and to describe the conditions necessary to cultivate that "diva", which is considered by some experts as a kind of missing link between the sticky traps and the snap-traps in family Droseraceae. Merging this awareness with our continuing experiments with other "snap-tentacle sundews" brought up suddenly, and to our surprise, a clear view of the relations between, and even the phylogeny of, the Droseraceae based on their easily recognized different trap structures. One more result was the understanding of the morphology and function of the odd yellow lens-tentacles on the leaves and bracts of D. hartmeyerorum Schlauer (Schlauer 2001). So now, almost four years after our DVD "Drosera: Snap-Tentacles and Runway Lights" (Hartmeyer & Hartmeyer 2006) was shown at the ICPS conference 2006 in Frostburg - which documents this work until 2005 - , it’s a pleasure for us to now provide a detailed summary of our comparative examinations on Drosera tentacles for CPN.

 Drosera tentacles in literature since 1875

      Talking about the ability of some species of carnivorous plants to catch prey with active moving traps has been of high interest for a long time. Therefore, it is actually amazing that the easily observed fast movement, and the differing morphology, of marginal Drosera tentacles, especially with several eye-catching pygmy Drosera (section Bryastrum, Lamprolepis), has been badly neglected, or simply ignored -- even with the publication of new species in recent literature -- at least until our publication in early 2005 on a sundew possessing a true snap-mechanism (Hartmeyer & Hartmeyer 2005). The scientific book "The Carnivorous Plants" (Juniper et al. 1989) mentions that several species exist which develop rapid moving, glue-less marginal tentacles (e.g. D. burmannii), putting them into a context of evolution from Drosera tentacles to the snap trap of Dionaea. But as mentioned, even the old authors knew about the non-glandular tentacles of D. rotundifolia. Beside Charles Darwin’s first remarks in "Insectivorous Plants" (Darwin 1888), descriptions are mostly restricted to a few scientific publications in professional journals, such as Flora (1904), where C.A. Fenner (Germany) provided wonderful and very detailed botanical drawings of D. rotundifolia and other carnivorous genera (Fenner 1904). Investigations on the movement of Drosera tentacles and how close they are related to the trigger hairs of Aldrovanda and Dionaea have been published by Prof. Stephen Williams (USA) in the early 1970s (Williams 1976, 2002; Williams & Pickard 1974, 1979). Using minute electrodes, he measured action potentials produced at the head or at the junction between head and stalk by touching or chemical stimulation (i.e. with sodium chloride) of the tentacle head. The action potentials move down the stalk where they cause particular cell-groups to increase their inner pressure. Without any junction, the whole tentacle starts bending inward toward the lamina due to rapid cell expansion (called turgor-growth). Stephen Williams examined mucilage producing tentacles of D. capensis for his experiments. In the early 20^th century, Henry D. Hooker, Jr. (1917) used the fitting term "non-glandular bisymmetric tentacle head" and dedicated his research on "the mechanics of movement in Drosera rotundifolia" mainly to the bending stalks of its marginal tentacles. In 1972, H.W.J. Ragetli, M. Weintraub, and Esther Lo (Ragetli et al. 1972) described two tentacle structures for the Drosera trap "… bisymmetrical tentacles lying in the plane of the leaf blade and extending outward from the leaf margin; and radially or nearly radially symmetrical tentacles extending upwards from the upper surface of the leaf … composed of an oval shaped head, a cylindrical stalk, and a connecting zone". However, it was worth it to examine numerous species again, because our results show clearly that there are not only more, but even really amazing, adapted tentacles with clearly different morphology.

Drosera glanduligera, the snap-trap sundew

A new diversity of tentacles

         Already in the early state of our experiments with rapid moving marginal tentacles, but increasingly after our examination of D. glanduligera, our macro-shots and microscope pictures disclosed a clandestine diversity, demanding an even closer inspection. Amazed about different shapes, speed, and the spasmodic appearance even in closely related species, we realized that these results contain not only obvious information about adaptation and relationship within the genus, but also on the evolution of the whole family Droseraceae.

Figure A1 (additional): Drosera hartmeyerorum lens-tentacles on florescence and leaf base. Video-screenshot by I.&S.Hartmeyer.	Figure A2 (additional): Drosera hartmeyerorum lens-tentacles illuminated with a torch. Video-screenshot by I.&S.Hartmeyer.
Figure A3 (additional): Drosera hartmeyerorum lens-tentacles on contrasting red bract. Video-screenshot by I.&S.Hartmeyer.	Figure A4 (additional): Drosera hartmeyerorum lens-tentacles magnified. SEM picture by Dr. Regina Kettering.

Beside the "normal" symmetrical mucilage-producing tentacles growing upright on the lamina, we identified three different types of marginal tentacles. We also found additional non-glandular structures like the unique yellow-shining lens-headed tentacles at the base of the trapping leaves and at the bracts of the florescence of D. hartmeyerorum (section Arachnopus). The head consists of lens-like transparent giant cells focusing at a bright yellow cell-structure in the center. Reflecting the incoming sunlight, these lens-tentacles function like a yellow rear reflector (Hartmeyer & Hartmeyer 2006). Additional lens-tentacles, placed at intervals on the dark red bracts of the green florescence, look like a chain of runway lights from above, attracting insects (we found lots of baby grasshoppers) to walk or skip down the flower stalk into the dark red and sticky plant center from where bundles of lens-tentacles show the yellow lightning with high contrast. We also found different small 0.1 to 1 mm-sized emergences with a simple yellow hemisphere-shaped head on the trapping leaves and stalk of Australian D. indica, and translucent mushroom-like emergences with a waved flat head on a large African D. indica (Hartmeyer 2003; Hartmeyer & Hartmeyer 2006). We guess that the mushroom-like structures produce the sweet odor of that intensely scented plant. From personal communication, we know about some more different emergences on sundews which are still subject to further examination and have not been published yet.

Our examinations on the lens-tentacles are on YouTube:

        These findings produce an increased understanding about the connection of marginal tentacles with the other genera assigned to the Droseraceae: Aldrovanda and Dionaea. Their modified non-glandular tentacles should not be forgotten. These are the trigger hairs on the snap-traps of both genera that developed from Drosera tentacles, as Stephen Williams described in the 1970s. But, also, the spine-like teeth on the margin of Dionaea lobes are obviously a remnant of marginal tentacles which lost their (no longer needed) trigger function.

Three different types of marginal tentacles

Figure 1: Drosera scorpioides, T1 sticky marginal tentacles. Photo by E. Pöhlmann.

Concerning the rapid moving marginal tentacles of Drosera, we confine ourselves only to the three types of "modern" tentacles that all emerge with a very broad and powerful base (four to six times broader than the base of the cylindrical tentacle stalks on the lamina) lying in the plane of the leaf blade. The following definition was published in 2008 (Hartmeyer & Hartmeyer 2008; McPherson 2008). The first type of marginal tentacles (T1, present mainly on the leaves of erect growing species) possess a mucilage producing, glandular symmetrical head which is similar to the normal stalked mucus secreting glands on the lamina of that species, however, develops on an elongated stalk that emerges with a very broad base. This type of elongated marginal tentacle is typified on the lamina of D. scorpioides for example. Movement in this type of tentacle takes place by turgor growth.

        The second type of marginal tentacle (T2, present mainly on the leaves of basal rosettes) possess a bisymmetrical, non-glandular head that lacks a sticky mucus secretion, often looking somewhat like a shovel with a flat lower side and an elevated mostly red, rarely green colored cushion-like glandless cell structure on the upper side, which develops on an elongated stalk that emerges with a very broad base. The size and shape of the elongated head and stalk vary considerably within different species. This type of elongated marginal tentacle is typified on the lamina of D. burmannii for example. Important: T2- tentacles always lack secretions of glue and movement takes place by turgor growth.

Figure 2: Drosera burmannii, T2 + T1 tentacle pattern on adult plant. Annotated by S. Hartmeyer.

Figure 3: Drosera glanduligera, T3 + T1 tentacle pattern on adult plant. Annotated by S. Hartmeyer.

The third type of marginal tentacle (T3) is known only from D. glanduligera. The stalk consists of a broad lower section emerging with a very broad base from the margin of the lamina and, connected by a kind of junction, a very slender but longer upper portion that terminates in a bisymmetrical, non-glandular head that lacks sticky mucus. Evidently, the tentacle head contains a trigger mechanism, which when activated, causes the tentacle to move very rapidly. The slender upper part of the tentacle bends forward from the junction-like midsection so that the head of the tentacle snaps towards the middle of the lamina. The whole process takes place remarkably quickly; we measured 0.16 seconds. The process of movement and the morphology of T3-tentacles differ therefore from all other tentacles known from the foliage of species of Drosera. The mechanism by which movement takes place is not currently understood. It is possible that some turgor growth takes place in order for the tentacle to bend, however, the process overall is much more sophisticated and therefore D. glanduligera is (until today) the only sundew that moves with the same speed as the Venus Flytrap.

Figure 5: Drosera glanduligera, outstretched snap-tentacle with junction in the middle. Photo by S. Hartmeyer.

Figure 6: Drosera glanduligera, triggered snap-tentacle, bend at the junction. Photo by S. Hartmeyer.

Table 1. Distribution of snap-tentacles (T2 & T3) in Drosera sections (incl. seedlings)

Archaic and modern Drosera

During our experiments, it became clear that the ontogenetic development of the seedlings also deserves a closer investigation. Surprisingly, many seedlings possess leaf generations (which follow the cotyledons) with a minimum of three to five powerful snap-tentacles (T2) which emerge morphologically as outgrowth of the lamina, but many of the adult plants do not.

Unfortunately, we were not able to get seeds from D. arcturi, D. regia, and section Prolifera.  Therefore, these species were examined for snap-tentacles looking at the first leaves of small plantlets, which grew from root- or leaf-cuttings (D. adelae, D. regia, and D. schizandra), developed at the florescence (D. prolifera), or opened first from winter resting buds (D. arcturi). The results show clearly that juvenile plants of the oldest known species in the genus, D. arcturi and D. regia, never develop broad-based marginal tentacles (T1-T3). This confirms the assumption that the first sundews caught their prey only with simple glue-tentacles on the lamina, but had already developed the ability to roll the complete leaf.

Also, the species in section Arachnopus (D. hartmeyerorum, D. indica), and section Prolifera (D. adelae, D. prolifera, D. schizandra) seem to descend from archaic species because here we found no snap-tentacles. However, one exception in section Prolifera is D. adelae, which possess some tentacles at the leaf-tips which also emerge from a broad base and are able to move faster (45 – 90 seconds) than the other tentacles on the lamina; therefore, they show some similarity to T1 tentacles. This may be a sign that section Prolifera is somehow in between the archaic and modern Drosera, but its tentacle structure is more closely related to the modern section Drosera than to section Arachnopus. This would also subsequently confirm Jan Schlauer's decision (Schlauer 1996) to extract the Queensland sundews from section Arachnopus into its own section Prolifera. Unlike all species which developed elongated marginal tentacles to fix prey to avoid theft by clepto-parasites, the species in section Arachnopus adapted and optimized their tentacles obviously to increase the attraction of prey.

The tuberous sundews (section Ergaleium) deserve further attention. Neither seedlings nor adult plants possess snap-tentacles (T2 + T3), but mainly the erect species show typical broad-based marginal T1-tentacles. As we documented with D. macrantha (Hartmeyer & Hartmeyer 2006), when triggered, they bend with about the speed of D. burmannii. This is actually a very modern feature.

Without a doubt, something substantial must have happened in the past that triggered the development of powerful broad-based marginal tentacles to fix prey to the lamina, maybe to improve digestion, but more likely to complicate the theft of prey. With this important adaptation, the modern Drosera turned up, becoming so successful that nowadays all (examined) species world-wide assigned to the sections Bryastrum/Lamprolepis, Drosera, Lasiocephala, Meristocaulis, Phycopsis, Ptycnostigma, Stelogyne, and Thelocalyx descend from the first modern sundews, all starting their life as a seedling equipped with powerful T2 snap-tentacles. 

Therefore, we define all species which develop either T1- or T2- or T3-tentacles as modern Drosera and those without these broad based marginal tentacles as archaic Drosera (see Table 1).

As mentioned above, D. glanduligera develops very effective and unique tentacles. At a first glance, the fastest snap-mechanism of the genus seems to be the most sophisticated and, therefore, very modern. Using a magnifying glass, we observed that minute springtails were catapulted into the sticky center of the lamina after touching the unique snap-tentacle extending from the lamina -- like the alarm threads of some spiders. But strange enough, its seedlings start with first leaf-generations (after the cotyledons) that possess only T1 stalked glands, even the next two or three leaf generations show only gluey and intermediate (between T1 & T3) tentacles. Only from the fourth or fifth leaf generation on, the typical rapid moving snap-tentacles with the junction-like middle-part function are found. Therefore, the ontogenetic development is distinct from all other species (examined so far) in the genus and therefore D. glanduligera must be considered as the oldest of the described modern sundews.

 
Basal rosettes and erect plants

In section Bryastrum/Lamprolepis, for example, we find a lot of adult plants that possess very prominent snap-tentacles, but closely related species growing sometimes at the same site, do not. This confusion disappears immediately if the growing shape is taken in consideration, as shown in Table 2. This pattern is typical for the species assigned to the modern sections and shows clearly that snap-tentacles are very important for seedlings and basal rosettes, but obviously not for erect plants. If the fixing of prey to the lamina would have been developed only to improve digestion, it should be useful for both basal and erect plants, but if the fixation is a strategy to avoid the theft of prey - which happens frequently, for example, by ants - the strategy is only important for trapping leaves near the soil (i.e., seedlings and basal rosettes). Erect growing plants, like D. scorpioides or D. madagascariensis, but also erect-held leaves, such as on adult D. capensis or D. binata (all seedlings and juvenile plants possess T2-tentacles), catch mostly flying insects, for which their sticky T1-tentacles work much more effectively. Just a little contact with the tip of a wing is sufficient to paste and hold a mosquito. The speed of movement achieved by T2-tentacles may be impressive for a plant, but is certainly not dangerous for flying insects.

Drosera of the Northern Hemisphere

Figure A5 (additional): Drosera rotundifolia bisymmetrical tentacle head. USB-microscope picture by S. Hartmeyer

Figure A6 (additional): Drosera intermedia hibernacula. Video scrennshot by S. Hartmeyer

Drosera of the Southern Hemisphere

The predominate number of Drosera can be found in the Southern Hemisphere. D. regia in South Africa and D. arcturi in Australia/New Zealand are the oldest known species and most experts agree that the genus developed in the south, spreading north over the course of time. As mentioned above, only modern species can be found north of Australia and South Africa, and surprisingly, the shape of their T2-tentacles seems to provide further information about their history. However, most T2-heads on seedlings and young leaves are round, or in D. binata sometimes nearly square, so only the shape of heads on adult leaves should be compared.

Let’s start in Australia, where we find more than 100 of the approximately 180 species world-wide (estimated, because new species are frequently found). As mentioned above, D. glanduligera , due to its ontogenetic development, is probably the oldest of the modern Drosera. The greatest morphological similarity with the unique T3-tentacles of D. glanduligera, and therefore indicating the closest relationship, are the T2-tentacle heads of pygmy Drosera, where we find also very long slender stalks with minute round heads. Actually, we also found the fastest T2-movement in this section with D. ericksoniae. Five seconds to bend to the lamina is indeed fast, however, still a magnitude slower than D. glanduligera. The large dichotomous leaves of D. binata (section Phycopsis) possess only mucus-producing T1-tentacles, but seedlings and juvenile plants develop T2-tentacles until the lamina starts to divide. 

Drosera burmannii, with very prominent snap-tentacles, shows the largest rectangular heads in the genus, referring to the plant size. It was in the past often called the fastest moving sundew, but with 8-15 seconds measured it is not faster than many pygmys. The only species in Australia assigned to section Drosera showing rectangular heads is D. spatulata. But both D. burmannii and D. spatulata also grow further north in Asia and seem to be not typical of Australians. The clear majority there show round to slightly oval heads.

Most of the species growing in the Americas are assigned to section Drosera. Unfortunately, we had only few plants to examine, but it's interesting that several species like D. montana, D. capillaris, D. felix, but also young plants of D. ascendens and D. villosa (the last two species have only T1-tentacles when matured) show a somewhat more three-cornered than true rectangular shape, which seems to be an American feature. But, as in Australia, we find also a few species that do not fit. Drosera sessilifolia (section Thelocalix) is only hard to distinguish from D. burmannii in Australasia and its snap-tentacles move with the same speed. The recently discovered D. meristocaulis (section Meristocaulis) develops snap-tentacles and is surprisingly very similar to the Australian pygmy Drosera. Drosera rotundifolia growing only in colder climates from North America through Europe and Siberia to Japan shows more rectangular heads.

Figure A7 (additional): Drosera sessilifolia adult trap with snap-tentacles. Video-screenshot by I.&S.Hartmeyer

Figure A8 (additional): Drosera sessilifolia bisymmetrical tentacle head. USB-microscope picture by I.&S.Hartmeyer

Most of the African species are also assigned to section Drosera, but D. admirabilis, D. aliciae, D. cuneifolia, and most of the other examined plants possess large rectangular T2-tentacle heads like D. rotundifolia or D. spatulata. In this connection also, the rectangular heads of D. burmannii (Australasia) and D. sessilifolia (South America) should be mentioned again. A good example for the development of modern tentacles during the maturation of erect plants is the South African D. cistiflora (section Ptycnostigma) which has first round, later drop-shaped T2-heads as long as it grows as basal rosette, but only two or three leaf-generations after the erect growth started, only sticky T1-tentacles appear in their place.

If we summarize all of the observations mentioned above and combine them with scientific confirmed incidents, a surprising theory (to be confirmed by other scientific disciplines) can be expounded:

Once upon a time, 200 or maybe even 300 million years ago, archaic vertebrates, plants, and arthropods like millipedes, insects, mites, and spiders, were living in the tropical rainforests at the southern coastal regions of Gondwana, an area that corresponds today mainly with Antarctica, but also the southern tip of South America in the west and Australia in the east. Certainly different from today, but following the same principles of life as nowadays, food chains and a web of interdependence including mutualism and parasitism developed. This interrelation enabled a group of early angiosperms (flowering plants) to use the manifold arthropods for their sexual reproduction. Compared with pollination by wind, animal-assisted pollination is much more beneficial because only a fraction of the pollen is necessary if it is carried by animals specific to the females. As a result, the modern insect-attracting flowers and a new web of interdependences developed.

Today all known carnivorous plants are angiosperms, so it is only logical to consider that they also realized very soon that arthropods attracted by provided nectar are not only good to transport pollen, but are also compact protein-packs which can be used very efficiently as nutrients, providing a certain advantage on poor rainforest soils. Therefore, some angiosperms started to catch prey with sticky tentacles on the surface of their leaves (and others probably also with pitcher forming leaves) and learned in the course of time to produce digesting enzymes and glands to absorb the nutrients. Thus, the archaic sundews, looking probably somewhat like D. schizandra or D. regia started their successful existence.

About 130-140 million years ago, these archaic Drosera had conquered the whole south of Gondwana, but now some wasps decided to settle on the ground and to use wings only for their mating flights. As a result, one of the most successful genera on earth appeared: the ants. Without a doubt, the appearance of ants caused a kind of revolution for all organisms living on the ground. Even today, it can be frequently observed that ants, which are certainly also prey for Drosera, steal considerable amounts of prey that the sticky plants caught. So, with the ants, the quantity of nutrients became suddenly seriously reduced. The ability to roll in leaves to hold captured prey more effectively and to increase the surface for digestion was too slow to avoid the theft by the omnipresent ants (and/or other clepto-parasites). But as part of the never ending story in nature to become the fittest survivor, some Drosera developed marginal tentacles equipped with a powerful broad base and the ability to move rapidly enough to fix the prey effectively before predatory ants are able to steal it. With this successful adaptation, the modern Drosera saw the light of day. During the following millions of years, new Drosera species and even two new genera changed the sticky and snap-tentacle armed trap into a closing snap-trap (which is even more effective to prevent the theft of prey), prospered well and proliferated north.

Caused by the continental shift, Gondwana broke asunder into the new southern continents and as early as 100 million years ago, the oceans in between became so large that the now separated species on the divided landmasses developed independent from each other. Antarctica, the area where probably the first Drosera developed, became isolated in the polar region and the archaic life was killed by frost. Australia, the last continent that broke away from the east of Antarctica drifted toward the equator and carried so many Gondwana species, like the marsupials, that it is now the epicenter with the most descendants of archaic and modern Drosera. South America broke away from the west of Antarctica earlier, but different species like D. sessilifolia and D. meristocaulis, with obviously closely related plants in Australia, may be evidence for common ancestors in Gondwana. Also, in South Africa, which separated even earlier, D. indica and numerous modern species survived, including the most archaic D. regia, which has several similarities with the Australian (and New Zealand) D. arcturi.

When our first observations and investigations on the movement and different shapes of elongated marginal tentacles started, it was interesting from the beginning, but we never expected how much new information can be collected even today using a simple microscope. Most of our published results have meanwhile been confirmed and supplemented by other CP-experts and authors. However, as mentioned before, our conclusion supporting the so called "Gondwana theory" should soon be confirmed by other scientific disciplines.

Acknowledgements

The following persons, listed in alphabetical order, provided helpful comments or other assistance in the preparation of this paper: Greg Bourke, Richard Davion, Christian Dietz, Andreas Fleischmann, Anja & Holger Hennern, Regina Kettering, Elvis Pöhlmann, Barry Rice, Jan Schlauer, Stephen Williams, and some more anonymous contributors.

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