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June 2002Pitcher in the bogSome nectar, the right shape, and there’s no escape.Anita CarpenterA pitcher plant stands ready to catch a meal. © Donna Krischan |
How could a hungry insect resist? The sweet aroma permeates the northern Wisconsin air. Attracted to the alluring source, the insect discovers nectar drops clinging to the hooded margins of a boldly-patterned plant. The insect lands, drinks, and walks around and down inside the hood. The nectar meal is rich, nourishing and filling. The insect turns around to climb up and out but its path is blocked by a wall of downward-pointing hairs. The insect is now hopelessly trapped. Unaware of its fate, the insect turns around and walks farther into the plant. It reaches a smooth, waxy area where it can’t maintain a foothold, slips and falls into the water-filled abyss and drowns. The pitcher plant has snared another meal.
Pitcher plants live in wet, acidic, nutrient-poor environments. They survive by luring and trapping insects that become their source for nitrogen. Nine of the ten pitcher plant species found in the United States grow in the south. Only Sarracenia purpurea, the northern pitcher plant, thrives on sphagnum moss mats of northern bogs.
Northern pitcher plants are perennials, each with a rosette of eight-inch leaves that grow together modified into tubular water-holding traps about one to two inches in diameter. The green trap leaves, striated with burgundy veins, flare out prominently near the mouth and collect rainwater. A vertical lid or hood rises from the top. Each trap leaf has four zones. The upper zone, the hood, has all the accouterments (smell, nectar, bold pattern) to entice insects. It is also covered with downward-pointing hairs that encourage the insect's descent and block its ascent. The second zone, the upper third to half of the leaf, lacks hairs but is coated with smooth plant wax that impedes an insect's footing. The diameter of the opening narrows, restricting room for flight as an alternate means of escape. In the third plant zone, the waxy cuticle is absent and the unwaxed surface absorbs nutrients. Deep within the trap, the fourth zone has a mesh of more downward-pointing hairs which also prevent the insect's exit if it hasn't already drowned.
Most pitcher plants secrete digestive enzymes to breakdown the insect’s exoskeleton and release its nutrients. Our northern pitcher plants are the least efficient member of the family secreting weak digestive solutions, if they do so at all. Mostly they rely on the digestive activities of microbes - bacteria, fungi, protozoans, algae and other small microscopic organisms living in the rainwater soup that accumulate in the base of the plant. The microbes all eat and benefit from the trapped insects and ultimately provide life-giving nutrients to the host pitcher plant.
One might assume that every insect that finds its way into northern pitcher plants becomes trapped. Interestingly, two insect species, the pitcher plant midge Metriocnemus knabi and the non-biting pitcher plant mosquito, Wyeomia smithii, depend solely on pitcher plants, living in the "soup" for all of their lives except for a short-lived adult phase.
Northern pitcher plants blossom in early spring, before or just as new pitcher plant growth appears. A solitary nodding flower tops a leafless stalk or scape. Five burgundy-red sepals radiate from the stalk on top of five strap-like petals. Nectar-producing glands lie at the base of each petal. Many stamens surround the ovary and hang down under the petals so the whole structure resembles an open, inverted umbrella.
Attracted by color, nectar and smell, a bee enters the flower at the only visible parting of the petal curtain. As the bee passes through the curtain, it brushes over a stigma lobe, pollinating the flower with pollen it has carried from another pitcher plant.
As the bee walks around inside the flower searching for the nectaries, it picks up pollen that rains from the overhanging stamens. The bee drinks its nectar meal and exits by pushing aside a petal and taking flight from one of the wide umbrella edges, bypassing the stigmas on its way out.
After pollination and fertilization, the petals drop, but the red sepals and umbrella remain all season. The five-parted ovary swells and in autumn, brown tubular seed pods split along the five seams, shedding teardrop-shaped light brown seeds. The seeds must be exposed to cold temperatures to germinate the following year.
Pitcher plants are truly unique plants, worthy of slogging through soggy, boggy terrain for a closer look.
