Orchid embryology: Pollen tetrads of Epidendrum scutella in the anther and on the stigma

Abstract

The pollinium of Epidendrum scutella, both in the anther and on the stigma, was examined with the electron microscope. The sporoderm of the outer tetrads has a sexine and an intine while that of the inner tetrads lacks a sexine, and an intine is formed only after the pollinium is on the stigma. A fibrous wall layer apparently holds the tetrads together. The cytoplasm is filled with plastids, mitochondria, polysomes, vacuoles and vesicles of various sizes, and endoplasmic reticulum (ER) with narrow cisternae. The vegetative nucleus is oval in form and contains a large nucleolus. The generative nucleus is deeply lobed and contains a well-developed nucleolus. The generative cytoplasm lacks both plastids and mitochondria and has little ER. Dictyosomes are present as well as assorted vesicles. A pocket is present between the plasma membrane of the generative cell and the wall; it contains assorted membranes and ribosome-like particles. After the pollen is on the stigma the wall surrounding the generative cell begins to disappear and gaps develop in it.

A cytochemical study of pollinium development in Calanthe Masuca (Orchidaceae).

“…During subsequent growth of the microspores, the polysaccharide granules disappear, sug gesting the diversion of polysaccharide metabolism towards the growth and differenti ation of microspores and their walls. Varying reports regarding the presence and absence of starch grains in the pollen of Epidendrum species are available (Cocucci andJensen 1969, Blackman andYeung 1983). Increase in RNA content in the pollen is linked with the behaviour of the nucleoli (Knox et al 1971), since the nucleoli of the microspore posses greater amounts of RNA and proteins.…”

Microgametogenesis in Plumbago zeylanica (Plumbaginaceae). 2. Quantitative cell and organelle dynamics of the male reproductive cell lineage

“…Cytoplasmic diminution mechanisms appear to operate throughout development in Plumbago, as was also the case in tobacco (Yu et al 1992, Yu andRussell 1994a, c). This diminution of male cytoplasm apparently eliminates organelles in some Apiaceae (Weber 1988) and Orchidaceae (Cocucci andJensen 1969, Yu and; however, in the majority of flowering plant sperm cells, heritable organelles remain present (Russell 1991). That flowering plants may display similar trends in cytoplasmic reduction suggests that genetic control is exerted on male cytoplasmic expression; however, there is also an emerging evidence that the rate of cytoplasmic decrease may also be influenced by factors external to the plant, including environmental effects as noted by Yu and Russell (1994d).…”

Orchid Biology: Reviews and Perspectives, X

“…The variations in this PDU type may concern wall structure: (a) exine is present around all the tetrads with some differences between inner and outer tetrads as in Epidendrum scutella (Cocucci and Jensen, 1969;Dressler, 1981, Fig. 3.22.f); (b) exine is present only on the outside of pollen grains of external tetrads (Figs.…”

“…5-8). In Epidendrum scutella (Cocucci and Jensen, 1969) and E. ibaguense (Yeung, 1987b, c) ornamented exine is only present at the periphery of the pollinium and a foot layer is present in all tetrads. Orchis purpurea only has reduced exine on external tetrads (Fig.…”

“…Pollen tubes therefore await complete development of the female gametophyte. This may take as long as 3 months, as in Epidendrum scutella (Cocucci and Jensen, 1969).…”