A spherical gland surmounting a tapering stalk comprises the stalked gland, which is commonly called a tentacle. These glands in healthy plants are enveloped in a layer of mucilage-like material which glistens in the sunlight, hence the common name Sundew. In strong light or sunlight the tentacle glands in most species develop a deep red coloration, whereas in the shade they develop little or no red coloration. The relatively clear and stiff mucilage which surrounds the tentacle gland presumably acts as a visible and odoriferous lure as well as the trapping medium. When an insect alights on a leaf and comes into contact with a gland, it is quickly mired in the thick mucilage. As the insect attempts to pull away one of its appendages, the mucilage is drawn out into thin threads. This commotion induces the glands with which it is in contact to produce impulses which travel to other tentacles. The impulses trigger the secretion process in other tentacle glands, resulting in the release of additional fluids. Simultaneously, the tentacles commence bending toward the prey. Eventually the flexing tentacles reach the prey, forcing it down to the surface of the leaf where it is bathed in fluids. The prey apparently drowns in these fluids. In some species the leaf also folds around the prey after its capture. With the completion of digestion and absorption the tentacles return to a vertical position again and the leaf blade, if folded, opens, exposing the undigestible remains of the prey.
The tentacle gland is exceedingly complex. A few of its multitudinous functions
Fig. 4-1 Drosera capensis plant. Older leaves form an apron around the base of the plant.
include: generation, acceptance and conveyance of stimuli resulting in the bending movements of the tentacles and increased secretion of fluids by the tentacle glands. If a deglanded tentacle is directly stimulated no movement results, indicating that the gland is necessary for the generation of an impulse. If a nearby tentacle is stimulated and produces an impulse, the glandless tentacle will bend in response to the stimulus. In addition, the gland secretes digestive enzymes, mucilage, water and probably an odiferous substance, and absorbs the digested materials.
Bending of tentacles is the result of differential growth. For example, if a tentacle flexes to the left, the rate of growth is greater on the right side of the tentacle than on the left side. The increased rate of growth originates at the base of the tentacle and progressively moves toward the top. When digestion and absorption have been completed, the tentacle assumes its former orientation by reversing the differential rate of growth. After tentacles have reached the limit of their growth, they lose their ability to bend in response to a stimulus; a tentacle can usually undergo this process 3-4 times. A Drosera leaf can capture and digest more than 3-4 insects because not all the tentacles are involved in capturing and digesting a single insect, particularly on large leaves. It is not unusual to see the remains of half a dozen or more trapped insects on a single leaf.
The number of tentacles involved in the capture of an insect is influenced by the size and strength of the prey. Size and strength of an insect will obviously determine the intensity of its struggle. As the intensity of the insect's struggle increases, the strength of the impulses produced also increases. More tentacles are stimulated as the impulse becomes stronger.
Stimulation of tentacles in nature is usually effected by insects, but tentacles can be stimulated artificially by physical contact with any object. Darwin discovered that as little as 0.000822 milligrams of matter will stimulate a tentacle gland. The crucial factor in stimulation of a gland is the contact of the stimulating agent with the gland surface. The tentacle gland is enveloped in a layer of mucilage: therefore, for stimulation to occur, the stimulating agent must penetrate the mucilage to reach the gland surface. If the gland was not protected by the mucilage, rain and dust could cause stimulation of the gland and result in a waste of energy due to useless bending. The layer of mucilage protects the gland and reduces the incidence of nonproductive responses.
The initiation of the bending of the tentacle is, at times, visible within a few minutes after stimulation. All stimulated tentacles will usually complete their bending within 18 hours of excitation. A tentacle stimulated artificially by physical contact will bend and straighten out within a day, whereas if suitable prey is the stimulating agent, the time interval between bending and straightening out is 1-2 weeks.
In addition to physical stimulation, the tentacles will also respond to both certain chemicals and hot water. Some substances such as creatine, a nitrogenous substance, when placed on the tentacle gland, will be digested without any visible movement of the tentacle.
Based on their response to stimuli and size, the tentacles of Drosera fall into three categories: marginal, interzonal and discal. These three general groups of tentacles occur in all species of Drosera which have been studied. The marginal tentacles are located along the periphery of the leaf blade and are usually the longest. Those found in the center portion of the leaf are known as the discal tentacles, and are usually the shortest. The interzonal tentacles are located between the marginal and the discal tentacles. They are of intermediate length, longer than the discal but shorter than the marginal.
The three groups of tentacles react differently to stimulation. The marginal tentacles will respond to direct stimulation; that is, the tentacle whose gland is stimulated will bend. It has been reported that they can complete their bending in less than one minute, a speed we have never observed. A stimulated marginal gland does not send an impulse to surrounding marginal tentacles. The initial bending of the marginal tentacles is nastic, which means that upon stimulation they will bend toward the discal gland area. After the nastic response, the bending of the marginal tentacles becomes tropic: the tentacles that were bending toward the discal gland area adjust their bending so that they move toward and touch the struggling insect, the source of stimulation. These responses are best observed on the leaves of Drosera filiformis var. filiformis; D. filiformis var. tracyi; and D. intermedia. The function of the marginal tentacles is to bring the prey in contact with the discal glands. At times the marginal tentacles will bend in response to impulses received from stimulated discal tentacles if the impulse is sufficiently intense. Impulses which evoke a response from the marginal tentacles are usually produced by strong and/or large insects. Often when an insect has been captured by the discal tentacles, the marginal ones commence to bend, but before completing their bending they will straighten out. Apparently the impulses generated by the discal glands were not continuous or of sufficient intensity to completely stimulate the marginal tentacles. Marginal tentacles stimulated by impulses from the discal glands respond less rapidly than those stimulated directly. They usually return to their original orientation in about a week.
The interzonal tentacles respond as rapidly to direct stimulation as the marginal tentacles do. In response to indirect stimulation, the interzonal tentacles react faster and more vigorously than the marginal ones. To direct stimulation the reaction of the interzonal tentacles is nastic, whereas to indirect stimluation the reaction is both nastic and tropic.
When stimulated directly by prey the discal tentacles do not respond by bending; they send impulses to surrounding discal, interzonal, and marginal tentacles, thereby stimulating them to bend, and resulting in their glands being forced onto the prey. The reaction of the discal tentacles is always tropic. They bend directly toward the prey after receiving indirect stimulation. These tentacles complete their bending from within 1-24 hours.
In some species such as D. capensis the whole leaf will fold to enclose the captured prey, presumably to allow more glands to contact the victim and so maximize the digestive process and as well as to form a saucer to maintain digestive juices in contact with the prey. (Fig. 4-3)
Other glands, the sessile or stalkless glands, are found on practically the entire surface of the plant excluding the roots. Apparently these glands are capable of absorption. Those located on the adaxial surface of the leaf and on the tentacle stalk may assist in the absorption of digested prey. Another possible function for these glands is that of guttation, excretion of water.
Drosera secretes enzymes, digests prey and absorbs the products of digestion intrinsically but is assisted by bacterial breakdown of prey.
In order to discuss the culture of Drosera in an organized fashion, it is convenient to group the plants according to environmental similarities into pygmy Drosera, tuberous Drosera, temperate Drosera, and tropical Drosera.
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