H. F. Linskens

123 phase and hence have no direct bearing on the retention time of solutes. However in gas-solid chromatography, a considerable quantity of the mobile phase may be adsorbed on the surface of the stationary adsorbent which diminishes the column's effective length and ability to retain solutes. In this respect helium has been found to be preferable to most other gases (GREENE and Roy, 1957) because it is adsorbed to the least extent. 3. Packed columns offer a considerable resistance to flow, which may create a pressure differential between inlet and outlet of sufficient magnitude to cause an unfavorable flow rate through a significant length of the column. A reduced inlet/outlet pressure ratio can be obtained by using light molecular weight gases toward which the column packing shows the greatest permeability. The flow rate of the mobile phase is normally adjusted by altering the column inlet pressure, for which purpose commercial pressure regulators of sufficient accuracy are available. Quantitative measurements of the flow rate can be made by a number of methods, including rotameters, orifice meters, soapfilm flow meters and displacement of water. The former two methods are the most con­ venient but the least accurate; moreover they create a back pressure and are temperature dependent whereas although the moving soap bubble is cumbersome to employ and unusable for continuous readings, it is preferred when the highest accuracy is required.

Pollen transmits the male genetic material in sexual reproduction of all higher plants. This same pollen is also well suited as a research tool for studying many patterns of plant and animal metabolism. In addition, an increased knowledge of pollen may help plant breeders accelerate efforts to improve the world's food and fiber supply. This volume focuses upon pollen biology and chemistry; it attempts to inte­ grate these facts with management practices involved in pollen applications. People have long been involved with pollen. Pollen applications are recorded in the rites of ancient civilizations (see Frontispiece). From the earliest times many benefits have been attributed to the inclusion of pollen in man's diet; also, since the mid-19th century air-borne pollen has been recognized as detrimental to many people's health. Disciplines concerned with man's cultural history and the earth's changing ecology find pollen a particularly useful and accessible tool. Identifiable parts of pollen have survived over 100 million years. But most books dealing with pollen are generally concerned with the identification of the plant source, an aspect of the science of palynology; other books emphasize the natural vectors transmit­ ting pollen, the pollination mechanisms. Very few works include the biochem­ istry or biology of pollen. Yet extensive studies by physicians, as well as plant breeders and apiculturists, have contributed a sizeable body of research relating to pollen.