The study of orbicules could be particularly rewarding since they offer a window on several biological issues on the borderline between gametophyte and sporophyte, such as the ratio of sporophytic and gametophytic genetic control in the development of the sporoderm, the function and chemical composition of lipidic fractions of tapetal origin in flowering plants (Piffanelli et al.
In general, mature orbicules are readily visible on the tapetal membrane (remnants of tapetal cells after programmed cell death) and are chemically inert to any preparation method because of their sporopollenin composition.
When orbicules are present, they are usually abundant and cover the tapetal remnants on the inner locule surface (Fig.
It has been reported that  orbicules originate in the cytoplasm of the tapetal
cells as lipoidal pro-orbicular bodies that accumulate below the membrane and eventually extrude to the cell surface (facing the locule) where they provide sporopollenin precursors for exine formation.
micro]m and are rarely embedded in the tapetal
Elaioplast is the final stage of tapetal plastid development (Fig.
First, in strictly anemophilous species, such as Lolium perenne, plastids first develop into chromoplast, later in elaioplast, and then are resorbed with the other tapetal cell components (Pacini et al.
Second, in entomophilous species, elaioplasts produce large amounts of lipids in their stroma, which are released into the cytoplasm through the plastid envelopes prior to tapetal plasma membrane disruption (Heslop-Harrison, 1968; Dickinson & Lewis, 1973; Pacini & Juniper, 1979b; Reznickova & Willemse, 1980, 1981; Reznickova & Dickinson, 1982; Keijzer & Willemse, 1988b; Murgia et al.
In Hypoxis, some tapetal cells remain at the edge of the locule in the late free microspore stage and give the appearance of a secretory tapetum [ILLUSTRATION FOR FIGURE 61 OMITTED], but earlier stages show the plasmodium [ILLUSTRATION FOR FIGURES 57-60 OMITTED].
In the secretory type, the pollen mother cells completely fill the centre of the anther locular cavity, surrounded by a layer of tapetal cells usually one cell thick [ILLUSTRATION FOR FIGURES 50, 72 OMITTED].
Tapetal cells pass through a number of cycles of secretory activity and redifferentiation, which is rather unusual for plant cells (Rowley, 1993).
Tapetal cells have an essential nutritive role for the developing microspores.