TINY TOUGHIES OF THE PLANT WORLD

Except for those of us who keep bees or suffer from hay fever, we probably seldom think about pollen, but these botanical male sexual gametes are an extraordinary and vital part of the plant world. Clearly, as with virtually all life, reproduction is fundamental, especially sexual reproduction that mixes the genes and drives Darwinian evolution and “fitness”. Most of us have learned about that at school but many fewer of us have had the opportunity to look at these tiny botanical marvels that drift on the breeze or are transferred by bees and a whole gamut of other vectors to enable cross-fertilization.

This is a superb public domain image made up from scanning electron microscopy at Dartmouth Electron Microscope Facility, Dartmouth College in New Hampshire, USA. The image magnification is ca. x500, so the bean-shaped grains of the golden-rayed Japanese lily Lilium auratum are about 50μm long. Others in the picture are sunflower Helianthus annuus, morning glory Ipomoea purpurea, Greek mallow Sidalcea malviflora, evening primrose Oenothera fruticosa (red triangular towards top right) and castor-oil-plant Ricinus communis.

Pollen grains, tiny though they are, are surrounded by a capsule, called an exine, made of one of the toughest materials known in biology. The exine is made of sporopollenin, a polymer the name of which combines pollenin and sporonin described by pollen scientists in the 19^th century. Sporopollenin is unique in nature and its toughness derives from highly cross-linked microscopic lipid (fat) units in which there are many apertures, as opposed to the more usual protein or polysaccharide polymers found in shells and nuts. Because they are so tough – they persist in geological samples that are 450 million years old – sporopollenins are difficult to analyse and decipher with regard to their molecular structure. They have many properties that could be of great use to materials scientists and to society such as resistance to sunlight, elasticity, antioxidant qualities and mechanical resilience. Candidates for using pollen exines and sporopollenin include ion-exchange chromatography, metals remediation, microcapsules for drug delivery, oil sequestration, and as microreactors and micromotors. First they must be extracted, though.

Pollen is extraordinarily variable with every plant species producing identifiably different grains with respect to their microscopic topography and morphology, and they vary in size from 5 to 100µm – a micrometre (µm) is a millionth of a metre. This enables us to look far back in time and determine what plants were growing where from the pollen fossil record.

You can read much more about this in the Royal Society of Biology’s magazine The Biologist, volume 66 number 4, pages 8–11. It is a fascinating topic for anyone interested in biology, ecology, materials science and archaeology.

Betts Ecology’s close encounters with pollen are mainly to ensure that the native wild grasses and forbs on our greenspace have the chance to flower and produce their pollen and resulting seed before they are cut down. It is not the pollen that is fragile: it is the plants and flowers if they are not allowed to mature and thereby protect and enhance the biodiversity of our semi-natural habitats.

 © Betts Ecology