Sexual Reproduction

Abstract

As discussed in Chapter 1, plant life cycles may be separated into two different phases, based on a change in the relative chromosome number from diploid in one phase to haploid in the other phase (Fig. 1–1). The diploid phase produces haploid spores by meiosis, and the spores give rise to a haploid organism, the gametophyte, that produces gametes. Union of gametes (fertilization) results in the formation of a new diploid organism, the sporophyte, which is the conspicuous generation in the vascular plants. In animals, the diploid organism usually gives rise directly to haploid gametes that unite and restore the diploid condition without a separate haploid phase. Sexual reproduction usually brings to mind the production and union of the gametes and the specialized organs concerned with this function. In the broad sense, however, the process must also involve the production of spores — in short, the entire life cycle of sexually reproducing organisms.