A Study of Factors Affecting the Germination of Alfalfa and Safflower Pollen

Pollen grains serve a very important role in the life cycle of flowering plants, in that they are involved in the transmission of inherited characteristics from generation to generation. Because of their small size the study of pollen grains has been necessarily associated with the development of the microscope. Not until the middle of the seventeenth century, when Hooke gave the world the compound microscope, was an instrument available with sufficient power to adequately reveal the shapes of pollen grains. However, two of Hooke\u27s contemporaries, Malpighi and Grew, who used a simple microscope, are generally recognized as the co-founders of pollen grain morphology. Nearly a century later Kolreuter discovered that pollen grains possess two distinct coats and observed that in some species the outer coat is elastic and often has spines and other sculptures. He also noticed that when pollen grains get wet the inner coat protrudes through apertures in the outer coat. The mechanics of fertilization in plants were discussed by several researchers during the middle 1700\u27s and early 1800\u27s. It was found that pollen grains germinated after making contact with the stigma, and subsequently, pollen tubes grew down through the styles and into the ovules. In the middle of the eighteenth century Gleichen suggested that pollen contained spermatozoa and had to burst in order to discharge them and effect fertilization. Needham in about 1740 discovered that many pollen grains, upon being brought into contact with water, expand, extruding papillae at their pores, and eventually burst, discharging their contents which appeared to consist of a viscous fluid charged with granules. He believed these granules to be the fertilizing materials which when discharged on the stigma, made their way through channels in the style to the ovules. Turpin in 1820 was the first to draw attention to the formation of the pollen tube. He stated that the pollen wall consists of two layers which he called the exhymenium and the endhymenium . He concluded that when a pollen grain is placed in water the outer membrane ruptures and the inner membrane expands, forming a thin-walled intestine-like tube bearing its exceedingly small granules. He assumed that these granules were endowed with independent movement. In an attempt to find out how pollen tubes arising from the small pollen grains are able to traverse long styles, Amici, in approximately 1830, discovered that the stylar tissue provides nourishment for the growth of pollen tubes. His findings suggested that pollen might be induced to gerninate and grow on artificial media of the proper nutrients were present. A knowledge of pollen grain structure and physiology is often very useful to a plant breeder, whose work involves artificial pollination. Crosses are frequently desired between plants whose flowering periods do not coincide. Pollen storage is one possible means by which such a cross might be effected. In situations involving pollen storage periodic assays of pollen viability are necessary. One of the simplest tests of pollen viability involves the germination of pollen on artificial media. Many of the difficulties encountered in obtaining inter-specific and intergeneric hybrids are traceable to problems involving pollen germination or pollen tube growth. Pollen may fail to germinate on foreign stigmas; the growth of the pollen tube may be too slow to reach the ovule in time to effect fertilization; or the pollen tubes may burst in the styles. In some instances these barriers to fertilization have been overcome by altering the stigmas or styles physically or by influencing the rate of pollen germination and pollen tube growth by manipulating environmental conditions or by using certain chemicals. Attempts to artificially germinate the pollen of different plant species have met with varying success. In the family Gramineae, for example, pollen has generally proved very difficult to germinate, whereas pollen from many members of the cucurbit and legume families germinates quite readily. In the present study two species, representing opposite extremes in pollen germinability under artificial conditions, were compared under varying environmental conditions and on a wide variety of media