ABSTRACT
The reproductive cycle of angiosperms includes two generations, sporophytic (diploid) and gametophytic (haploid). The gametophytic generation is extremely reduced; the male (pollen) consists of a vegetative cell and two sperm cells, and the female (embryo sac) of seven cells, including the gametic cells (egg cell and central cell). The union of sperm cells with the female gametes (double fertilization) requires the development of a pollen tube to convey sperm cells through the pistil to the embryo sac. Pollen-tube development results from a continuous interaction with the transmitting tissue of the pistil; the interaction is controlled by genetic systems which prevent growth after either cross-or in-breeding (see De Nettancourt 1977). The transition from sporophytic to gametophytic generations occurs via the meiotic process. This comprises a series of coordinated developmental stages of the sporocyte, also correlated with the development of the surrounding sporangium tissues. Any developmental abnormality during meiosis may result in gametophytic sterility. A factor usually associated with abnormal meiosis is polyploidy. The pollen of polyploid plants, especially triploids, shows a variable degree of pollen sterility (see Carroll 1966). Female sterility is less known because it is difficult to evaluate.
