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Simcha Lev-Yadun

ABSTRACT

The question of the spread of an aposematic or of an otherwise visually defended plant type within a non-aposematic or a visually non-defended population is a long-standing enigma that has received considerable theoretical attention. However, the spreading of aposematic or otherwise visually defended plant genotypes within a non-aposematic or a non-visually-defended population has never, as far as is known, been shown or studied in nature in wild plant populations. This study investigates the loss of the various simultaneous types of defensive coloration in the spiny thistle Silybum marianum by a mutation that occurred independently and found in 13 wild populations in Israel. Mutant plants have plain leaves rather than leaves of the zebra-like wild-type, which has a white network of stripes on the upper leaf surface. The mutants never spread beyond several dozen meters and usually only over several meters. The mutation has a simple developmental origin, since the white variegation is the result of small air spaces formed between the epidermis and the photosynthetic parenchyma (causing no loss of photosynthetic capacity in white areas), and the mutants have no such subepidermal air spaces. In order to examine the possibility of establishing a pure population lacking this type of defensive coloration, seeds of mutant plants were collected from two wild populations where they grew mixed with the wild-type and a pure mutant population was selected and maintained for 4 years. Thus, 13 cases of very restricted spread of the visually defenseless mutant demonstrate the probable contribution of the variegation to plant fitness, supporting the hypothesis that conspicuous leaf variegation functions as defensive coloration.

Simcha Lev-Yadun

ABSTRACT

While plant mimicry by animals to make them cryptic from both prey and predators has received significant attention, the reverse situation, i.e. animal mimicry by plants as defense from herbivores, has been paid dramatically less. Here, in an essay intended to both stimulate and intrigue, I describe the various proposed types of defensive animal mimicry by plants, discuss the few published experimental tests of this hypothesis, and propose some future directions of research of defensive animal mimicry by plants. Animal mimicry by plants as defense from herbivores comprises two general types: direct animal mimicry, and mimicry of cues about animal action. The direct type includes bee, wasp, caterpillar, ant, aphid, beetle, butterfly, eye, and snake mimicry. The animal cue mimicry includes tunneling, chewing damage, spider web or arthropod silk, animal dung-shaped plants, and carrion and dung odors. These defenses include Batesian mimicry, masquerade, a mixture of Batesian mimicry and masquerade, and probably also perceptual exploitation. As an overlooked phenomenon, this area of evolutionary ecology has good potential for interesting finds.

Simcha Lev-Yadun

Firewood is a vital energy source for cooking and heating in traditional societies worldwide. During the past century, increasing human populations have depleted many previously available resources, resulting in severe shortages of firewood in many regions, especially in arid zones. Here, I describe the use as a source of firewood for a local semi-nomadic Bedouin village of several dozen families and the fate of a 0.3 km2 abandoned fiber-crop plantation of Agave sisalana Perr. which is more than 50 years old in the Negev Desert, Israel. The amount of firewood extracted in the last decade equals several thousand local wild shrubs, which, in the current vegetation density, grow in a radius of several square kilometers. Harvesting the plantation for about 20 years almost fully exploited the plantation. Because the Bedouins do not replant plants used only for firewood, this source is not sustainable despite its biological potential to be so. However, because Agave are CAM (Crassulacean acid metabolism) plants, they can grow under very arid conditions, and because they are well defended from grazing by thorns and poisons, they may be planted as a source of firewood in various arid regions where other plants will not sustain.

Simcha Lev-Yadun

Trichomes are known to have many functions, including protecting plants from excess sunlight, improving water economy, salt secretion, defense from herbivores, and signaling to animals. Additional anti-herbivore functions of trichomes, especially in coastal and desert habitats, are reviewed and proposed. Many sand-dune and sandy shore plants are white, whitish, or silver-colored because of white trichomes, because of sticky glandular trichomes to which sand grains and clay adhere, or because of light-colored waxes. The common explanation for this coloration is that it protects from irradiation, and that in addition, the glued sand defends them from abrasion by moving sand. This coloration was also proposed to camouflage the plants from herbivores. Similar coloration in animals that live in white, snow-covered habitats or light-colored sand or other soil substrates is commonly referred to as camouflage, and the same logic may also apply to plants. It has also been proposed that white plant surfaces undermine the camouflage of herbivorous insects that have other colors and expose them to predation. Three novel defensive mechanisms are proposed here: (1) because dust is a strong insect repellent and is lethal to insects, attached soil particles (especially clays) may defend plants with sticky glandular trichomes from insect herbivory; (2) in dicotyledonous plants that have sticky glandular trichomes, the attached sand may defend from herbivory by mammals by causing teeth wear as do phytoliths (silica bodies) of grasses; and (3) white coloration of leaves and branches may mimic fungal infestation. Direct experimental data for the functionality of these defensive mechanisms are missing for many of the old and all new hypotheses, but there are many indirect supporting indications.

Simcha Lev-Yadun

Twenty-one wild spiny or thorny plant species growing in Israel have been found so far that are conspicuous because of white stripes and spots found on their leaves. Twenty of these species occupy open habitats, and only one is a climber (Smilax aspera) that is found in both shady and open habitats. I propose that these spiny, thorny, or prickly conspicuous plants form a defensive Müllerian mimicry ring. The genus Launaea (Asteraceae) includes several species that are both white variegated and spiny or thorny (a defended Müllerian mimicry ring), and four non-thorny but variegated plants (a Batesian mimicry ring). I propose that these four species that form a non-defended Batesian mimicry ring enjoy the indirect protection of both their co-generic spiny and thorny species and also of defended plants from other taxa. The long history of the considerable impact of grazing in this arid region seems to have selected for this character.

Simcha Lev-Yadun

Some additions, modifications and deletions to the terminology used in bark anatomy are suggested. The anatomical and developmental basis for these suggestions is briefly described.

Simcha Lev-Yadun

Wounding and partial girdling of stems of Rhamnus alaternus L. inhibited the formation of vessel wave patterns that are normally formed in the secondary xylem of this species. The patterns returned to normal in some of the trees in the first or in the second year after wounding. The possible hormonal regulatory mechanisms involved in the histological changes are discussed.

Simcha Lev-Yadun

The relationship between growth-ring width and ray size and number was studied in five 24-year-old trees of Pinus halepensis Mill. and five 16-year-old trees of Pinus pinea L. All trees of both Pinus species showed a gradual tendency for an increase in ray height, from an average of less than 4 cells near the pith to 7 or 8 cells in the outer rings. Ray number decreased from more than 70 rays per mm2 near the pith to about 40 rays per mm2 in the outer rings. No significant correlation was found between growth-ring width and the number of rays per mm2 or height of rays for three out of five trees of P. halepensis or for any of the five P. pinea trees. I conclude that there is no general direct relationship between growth-ring width and ray number and size.

Simcha Lev-Yadun

Circular vessels differentiate in the secondary xylem of the short stem of Arabidopsis thaliana at the rosette level, where many inflorescence s grow following the repeated cutting of developing inflorescences over several weeks. The circular vessels differentiate adjacent to developing buds, but are not found in branching regions of the inflore scence s.