Planta (1984)162:385-391 Planta Springer-Verlag 1984 Ultrastructure of the sperm of Plumbago zeylanica II. Quantitative cytology and three-dimensional organization* Scott D. Russell Department of Botany and Microbiology, University of Oklahoma, Norman, OK 73019, USA Abstract. Pollen grains of Plumbago zeylanica L. were serially sectioned and examined using trans- mission electron microscopy to determine the three-dimensional organization of sperm cells within the microgametophyte and the quantity of membrane-bound organelles occurring within each cell. Sperm cells occur in pairs within each pollen grain, but are dimorphic, differing in size, mor- phology and organelle content. The larger of the two sperm cells (Svn) is distinguished by the pres- ence of a long (approx. 30 gm) projection, which wraps around and lies within embayments of the vegetative nucleus. This cell contains numerous mi- tochondria, up to two plastids and, infrequently, microbodies. It is characterized by a larger volume and surface area and contains a larger nucleus than the other sperm cell. The second sperm cell (Sua) is linked by plasmodesmata with the Sv, , but is unassociated with the vegetative nucleus. It is smaller and lacks a cellular projection. The Sua contains relatively few mitochondria, but numer- ous (up to 46) plastids and more microbodies than the other sperm. The degree of dimorphism in their content of heritable cytoplasmic organelles must at fertilization result in nearly unidirectional trans- mission of sperm plastids into just one of the two female reproductive cells, and preferential trans- mission of sperm mitochondria into the other. Key words: Angiosperm reproduction- Mitochon- drion (in pollen) - Plastid (in pollen) - Plumbago - Pollen - Sperm cell. * I:Russell and Cass (1981) Abbreviations: Sua= sperm cell unassociated with the vegetative nucleus; Svn= sperm cell physically associated with the vegeta- tive nucleus Introduction Although the sperm cytoplasm of angiosperms is hardly regarded as the expendable "sheath" that it was in the earliest reports of fertilization (Ma- heshwari 1950) and even into recent times (see re- view by Jensen 1974), its later influence on the development of the embryo is currently regarded as being, at best, variable and to some extent, taxon-dependent (Birky 1983). If one current mod- el of fertilization is correct, at least some, and per- haps most of the male cytoplasm is transmitted into the female reproductive cells during the pro- cess of transmitting the male nucleus (Russell 1983). While some components may have a tempo- rary influence on reproductive development, only the DNA-containing cytoplasmic organelles and the nucleus are expected to have a lasting effect (Gillham 1978; Hagemann 1979). Genetic evidence shows that the male contribu- tion of plastids and mitochondria in flowering plants can have a great impact on the cytoplasmic characteristics of succeeding generations in a number of different taxa (see review by Gillham 1978). The initial quantity of heritable organelles so transmitted appears to bias the outcome of cyto- plasmic assortment (see Birky 1983). Genetic re- combination in heritable organelles has also been demonstrated in some angiosperms (Gillham 1978) and when it occurs, paternal characteristics may become fixed within the genome of embryo organ- elles, even if there are relatively few heritable or- ganelles present in the transmitting sperm cell. The final genetic constitution of organelles within the embryo in either case is sensitive to organellar con- tent in a probabilistic sense (Birky 1983), and the paternal input, as variable as it appears to be in