COLLECTING AND ANALYZING POLLEN ON HUMMINGBIRDS
By Fernando Ortiz-Crespo of Quito, Ecuador (Note 1)
A hummer's bill may seem fairly smooth, but it usually carries pollen grains adhered to its surface. Pollen gets there as the bird visits its favorite flowers, and thus one can "read" a record of the bird's flower preferences by examining the grains found on and around its bill.
A pollen grain is a marvelous structure evolved by plants to protect and deliver the male gametophyte generation. Pollen grains develop and are borne on a flower's stamens, from which they are carried away by external agents, such as the wind or an animal (usually an insect or a bird). Individual grains are often deposited on the pistil of another flower of the same species of plant. There the grain germinates producing a long tube that penetrates the soft tissues of the pistil's stigma (the pistil's tip) and style (the pistil's thin stem), the tube eventually reaching an ovule within the flower's ovary and delivering the male gamete, thus bringing about fertilization and seed development.
To carry out this delicate and important function, a pollen grain is protected by a very hard outer cover, called the exine, with a very distinctive shape and texture that help in tracing a grain to the particular plant source from which it came. The exine is not only species-specific, however, but it is so resistent that, when pollen grains are deposited in the ground and covered by sediments, such as in the bottom of lakes, their exine cover might last for thousands or even millions of years.
Thanks to this, pollen is crucial for sediment dating studies and palaeoclimatic and palaeoecological analyses. Because of their extraordinary resistance to environmental agents, pollen grains must be subjected to strong clearing agents to be properly analyzed. The traditional pollen analysis treatment is a boiling acid mixture, in a process termed acetolysis, which eventually digests away the inner contents of the grains leaving behind the nearly transparent exine. The grain can then be photographed under the microscope.
However, more recently biologists have developed an easier method to analyze pollen grains, clearing them with a synthetic substance called polyvinyl lactophenol, far less dangerous and difficult to use than the boiling acid mixture.
However, a more direct way of observing pollen grains under high magnification without having to first clear them is to place each slide under a scanning electron microscope, a device that "bounces" a thin electron beam off the grain, which has previously received a thin layer of vaporized metal (gold) on its surface.
The metal on the grain's surface behaves like an electron mirror. Electrons reflected by the object are used to construct a TV-like image on a screen and to get 3-D quality photographs.
To obtain pollen from a hummingbird, all one needs is a few microspcope slides and a roll of double-stick tape. A small bit of tape is rubbed with the help of a tweezers against the bill's surface and on the facial feathers, and then the tape is stuck to the center of the glass slide. Once there, the sample -- protected against further rubbing and dust -- can eventually be treated for clearing and observation with one of the methods previously described.
REFERENCES Note 1. Boeke, J. D. and Ortiz-Crespo, F. I. 1978. Avian pollination studies: a simple scanning electron microscope technique. Science, 201: 167-168.
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