Pollination in Sarracenia

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From Pitcher plants III, Entomological News, Apr . 1908

That Sarracenia must depend upon insect agency to effect pollination of the blossom was recognized long ago by the botanist, and the structure of the flower indicates something of the method by with c this must be accomplished. Careful observation of the insect visitors of Sarracenia flava throughout its blooming season make it seem probable that in this species the method of pollination differs in some respect from the published accounts of this process in the genus Sarracenia in general. In flava, as in the other species of the genus, the style is a curious umbrella shaped structure, each of its five points being cleft, and the stigmatic surfaces are situated on little projecting points at the base of these clefts on the concave side of the open umbrella. The petals at the base form a close bell-shaped cover, spreading out and filling the space between the points of the inverted umbrella; and access to the nectar and pollen is possible only at one of five openings, situated just below the curled-up tips of the umbrella with their projecting stigmatic points.

An insect alighting on a petal enters the flower, turns at right angles in either direction to one of these openings, and in forcing its way through, if of suitable size, scrapes its back across the projecting point of the stigma. In flava especially, egress is exceedingly difficult except at the five entrance holes; and all of the numerous insects observed visiting these flowers emerged after considerable struggling through one of these orifices, almost invariable again scraping the stigma in their departure. In flava therefore, with its heavy closely-overlapped petals usually preventing the escape of the visiting insects except by re-passing the stigma, some special provision for preventing habitual self pollination by insect agency would be expected; and this seems to be provided for in the position the flower assumes upon its stem at various ages. When the bud first appears above the ground it is borne upright upon the stem; and as the blooming period draws near, the stem bends just below the bud, making a complete turn, so that when the flower opens, the style occupies the position of an inverted umbrella, catching and retaining the falling pollen, most of which is shed within twenty-four hours after the flower begins to open. The flower then begins again to change its position, often being very noticeably tilted on the stem by the third day, and eventually, long before the fall of the petals, it takes a position at right angles to the original one. These changes of position and structure of the flower are shown on Plate IX. The tilted and finally upright flowers naturally retain less pollen than the newly opened horizontal ones, and often the tilted flowers show little trace remaining of the abundant supply in the newly-opened blooms. As the blossoms remain fresh and continue to be attractive to insects for more than two weeks, it would seem that this change in position of the flower and the consequent spilling of the pollen decidedly favor cross-fertilization.

Ants are almost invariable present in the flowers, attracted by the abundant nectar oozing from the ovary, but they are probably of little importance as pollenizers. The ant most abundant in flava at Summerville is identified by Prof. W. M. Wheeler as Tapinoma pruinosa Roger. Ants, wasps, and occasionally butterflies visit the outside of the flower; spiders, small beetles, and even the little green tree frogs which habitually occupy the leaves, are sometimes found ensconced in the flowers; but from the first opening of the flava blossoms in March, their most frequent visitor on sunny days is the honey bee, which being of suitable size to snugly fit the orifice, rarely enters or leaves without brushing the stigma. Most of these bees carry loads of pollen, and do not seem to exercise any choice, relative to the age of the flowers, in making their visits. Much smaller bees, Augochlora and Osmia, are much less frequent visitors and on account of their size usually escape contact with the stigma. At rare intervals a bumble bee may be seen forcing its way into the flower by the usual path, but this insect proved so infrequent a visitor that it can scarcely be considered of much importance as a pollenizers [sic]of flava.

One other insect, however, of suitable size to effect pollination, an insect always associated with Sarracenia, is a constant visitor to the blossoms. The Sarracenia fly, Sarcophaga sarraceniae Riley, habitually resorts to the blossoms as well as to the leaves, perhaps more for shelter than for food. At night and on cool, windy, or rainy days these flies crowd into the blossoms, sometimes to the number of three or four to one flower; they are rough, bristly, and often yellow with pollen; they enter and leave the flowers by the only practicable path, the orifice just under the stigma, which they are of suitable size almost necessarily to touch in passing.

It has been suggested that the pitcher plant moths (Exyra) may be pollenizers of these flowers; but as flava at Summerville commences to bloom in March, and Exyra ridingsii, the species most intimately associate with it, does not appear until the middle of May, and Exyra semicrocea not until the middle of April, this can scarcely be the case. The color of the flowers and the fact that their fragrance becomes more noticeable toward evening indicated the possibility that night flying insects may aid in pollination. The following list includes all the daytime visitors found in sufficient numbers to indicate that they are habitual visitors to these flowers:

In Sarracenia flava:

Coleoptera:

• Chauliognathus marginatus Fabr .

Diptera:

• Sarcophaga sarraceniae Riley

Hymenoptera:

• Apis mellifera L
• Bombus pennsylvanicus De G.
• Augochlora (confusa Robt.?).
• Osmia sp.
• Halictus sp.
• Tapinoma pruinosa Roger.

The small brilliantly-metallic bees, Augochlora, and an Osmia, were also noted frequenting the blossoms of Sarracenia minor, which did not seem to be visited by the larger insects. Dr. Mellichamp has recorded a beetle, Euphoria melancholica as an occasional visitor to this flower. The size and structure of the flower, however, seem to indicate the small bees as the more suitable pollenizers.

From unpublished notes

Further observations strengthen the belief that for the smaller species (minor, psittacina) small bees are the more frequent visitors; that these too are frequent a the larger flowers (flava, sledgei, drummodii) to which bumblebees occasionally may be observed as visitors; but that Sarcophaga flies are also much in evidence, crowding the blooms in cool or rainy weather. Expanses of the blooming plants visited at night did not reveal any night attracted insects, and the gathering and prompt bagging of may blooms (sledgei), examination the next day, did not indicate the presence of night flying species. The presence of nocturnal insects among the prey indicates that the nectar-bait of the pitchers is fully operative throughout the night.

Similar observations upon blooming Darlingtonia—not so obviously specialized for insect pollination—indicated no night flying species regularly attracted to the flowers; that bumble bees were day visitors to a limited extent; the hawkmoth, C. lineata, was more than once seen flying from flower to flower; butterflies more apt to alight on the nectar secreting hood or fishtail, but occasionally to probe the flowers with their tongues (Neophaia menapia, rarely a Thecla); but butterflies are certainly not frequent visitors. Small spiders are almost invariably present; also in numbers, a small and very active thrip. Night visits to the expanse of blooming plants revealed no night visitors, which accords with their coloration and the absence of strong perfume. 

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