The north temperate forms of sundews are generally perennial plants, with several leaves forming a rosette. The plants have a short stem with fibrous roots. The leaves of the sundew vary in shape and size in different species, but the basic function and structural characteristics are the same for all species. The leaves are of two parts: a narrow petiole (leaf stem) at the leaf base, and a leaf blade modified into an adhesive trap.

Structure of Tentacle

A closer look at the sundew tentacles reveals that, unlike a usual plant hair which is of epidermal origin, the tentacle has a far more complex structure exhibiting all the elements of the leaf itself. It is composed of a tall, tapering stalk of multi-cellular structure, topped with two layers of glandular cells. A slender vascular strand can be seen in the stalk center to support gland-to-leaf linkage. Mucilage is secreted from the glands through their cuticular discontinuity. In addition to these stalked glands (tentacles), there are numerous sessile (stalkless) glands. These are found on the leaf surface as well as on the stalks of the stalked glands. Actual functionality of the tentacle, however, appears far more complex and diverse than its apparent structural manifestation. 

Detailed studies have been conducted on the behavior of sundew tentacles by various authors, including Charles Darwin and F. E. Lloyd. On the most widely known species of sundew, Drosera rotundifolia, the tentacles can be divided into three groups based on their response characteristics to stimulation. Those tentacles found on the periphery of the leaf blade -- usually the longest tentacles of all -- are known as the "marginal tentacles". Those near the leaf center are termed "central tentacles" and are usually the shortest.  Between the marginal and the central tentacles grow tentacles of intermediate height called "outer tentacles". By and large, a similar observation holds for behavioral characteristics in most sundew species, although some differences may be noted due to the particular posture of tentacles in different species with varying leaf shapes. Illustration

The marginal tentacle, when stimulated directly, always bends toward the leaf center. This type of bending motion toward a pre-determined direction, regardless of the direction of outside stimuli, is known as the "nastic" movement. The tentacle also responds to indirect stimulation -- an impulse transmitted from outer or central tentacles. When a direct stimulus is given to the marginal tentacles, however, only the stimulated tentacle will bend, i.e., the marginal tentacle does not transmit stimuli to other tentacles. This behavior immediately distinguishes the marginal tentacles from the other two groups of tentacles.

The central tentacle, on the other hand, does not respond to direct stimulation, like the other two groups of tentacles. When stimulated, it does send out impulses to the neighboring tentacles in a matter of several minutes to a few hours. When stimulated indirectly, the central tentacle bends toward the point of the stimulus. This behavior of the tentacle -- bending toward the direction which has a consistent correlation to the source of stimulation -- is known as the "tropistic" movement.

The outer tentacle, located in the middle zone between the marginal and the central tentacles, exhibits somewhat of a combined characteristic of these neighboring tentacle groups. The outer tentacle responds to the direct stimulation by exhibiting a rapid nastic motion, just like marginal tentacles. It will transmit an impulse to nearby tentacles as well.

For indirect stimulation, the behavior of both the marginal and the outer tentacles is more complex, typically showing an initial nastic movement followed then by a tropistic reaction, as if to correct orbital errors, as it were, to reach the prey. The outer tentacles are known to be more responsive and agile to the indirect stimulus than the marginal tentacles. The actual tentacle behavior may vary somewhat depending on where the stimulus has originated, on different species, and on growing conditions. For instance, the higher the temperature, the more likely the tropistic reaction is to dominate.

The mechanism of tentacle bending is not fully understood. The bending is precipitated by a sudden drop of cellular pressure on one side of the tentacle stalk. The resultant imbalance of pressure between the opposite sides of the stalk causes the tentacle to bend. The pressure differential first occurs near the tentacle base, and then gradually moves upward.

Sometimes a quite rapid tentacle movement can be observed in some species -- bending more than 180 degrees in less than 60 seconds -- with a proper stimulation (mechanical and chemical) under an optimal condition.

Eventually the gland secretions are switched from sticky substances to digestive juices. The prey, having forced down to the surface of the leaf by flexing tentacles, is often submerged in the digestive fluids and is suffocated. The bending motion of the tentacles allows the prey to be transported to the pre-designated center portion of the leaf blade where the digestion takes place. This brings about economies in quantities of digestive enzymes. As the digestion progresses, products of digestion are promptly absorbed into the leaf interior through the leaf surface and, in a matter of a few hours, begin to be carried away to the other parts of the plant (Komiya et al, 1978). Numerous, tiny sessile glands found on the entire surface and on the tentacle stalks also aid in the rapid absorption of the digested material.

After the absorption is completed, the tentacle returns to the original position. It is a reverse process of bending, again due to pressure differentials. This unbending is a slow process usually taking a day or more. It is believed the unbending is a growth phenomenon seen as the result of restoring the cellular pressure. As such, the same tentacle is measured to be greater in length by 10 percent or so after unbending. A given tentacle can repeat the bending and unbending only a few times before reaching the growth maturity.

For the most species in the temperate zone, a tall scape appears from the rosette center in the spring, which bears numerous flowers. Actinomorphic (radially symmetric) flowers have five petals and are typically small and plain. Most of the sundew flowers are believed to be insect-pollinated. Usually a flower opens for one day. If the pollination does not take place, the anther and stigma are brought together as the flower closes at the end of the day, thus effecting self-pollination. Many flower scapes are produced and flowering continues through the summer season for most species.

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