Generative Cells

Abstract

Sexual reproduction is one of the most important events in the life history of eukaryotic organisms. Throughout the process of algal and plant evolution, the mode of mating has changed from isogamy to unisogamy, and, finally, to oogamy. In parallel, the mating process has become progressively less dependent on the presence of external water. In algae, bryophytes, and pteridophytes, male gametes are flagellated sperms which move to eggs by swimming in external water. In some gymnosperms, male gametes are still sperms with flagella, but they do not reach eggs by swimming through external water; instead, pollen grains (male gametophytes encapsulated within a pollen wall) travel through dry air to the ovules; thus sperms must swim only a short distance across the fluid-filled archegonial chamber in the ovules. In the evolution of gymnosperms, male gametes lost flagella and motility and came to be delivered by pollen tubes, making it possible for male gametes to reach egg cells without being exposed to the extracellular environment. Gametophyte generation also evolved to become more simple. In bryophytes, gametophyte generation is a dominant phase. Pteridophytes keep gametophytes (prothallia) small while sporophytes become larger and more complex. In gymnosperms, gametophytes become extensively smaller. In angiosperms, the male gametophyte (pollen) is composed of only three cells, and the female gametophyte (embryo sac) is typically seven cells. Among the ten gametophytic cells, however, as many as 4 cells participate in double fertilization, which is specific to angiosperms. In this chapter, 14 selected figures illustrate various aspects of sexual reproduction in algae and plants.