Biology Reference
In-Depth Information
the first mitosis, 4-6 days before anthesis, the vacuoles decreased in size (Figure
5C) and starch was again stored (Figure 5D). Young pollen grains had no vacuoles
(Figure 5E) and numerous starch grains were present (Figure 5F). The second mi-
totic division producing the first tricellular pollen grains started some four hours
prior to anther dehiscence (Figure 5G) and one day prior to anther dehiscence
starch grains began to hydrolyze (Figure 5H). Mitotic divisions were not synchro-
nized within a pollen tetrad and single pollen grains with different numbers of
nuclei could be observed in the same tetrad resulting in the coexistence of bicel-
lular and tricellular pollen upon anther dehiscence.
Figure 5.
Microgametogenesis in Annona cherimola.
(A) Microspores
increase in size as vacuoles appear in the cytoplasm, (B) microspores at this stage do not have
starch grains. (C) Microspores following mitosis I; (D) as vacuoles decrease in size, starch grains are stored. (E)
Young pollen grains without vacuoles (F), which accumulated starch grains. (G) Close to the time of anther
dehiscence, the second mitosis occurs, the tapetum is completely degenerated and (H) starch is digested.
Longitudinal sections of anthers stained with PAS and Toluidine blue (A, C, E, G), and with PAS (B, D, F, H)
to show starch grains. Bar = 20
ยต
m.
In mature pollen, while intine thickness was similar around the pollen grain,
the exine was thinner or absent at the pollen aperture sites where contact points
between sibling pollen grains were established (Figure 6A). At these areas unester-
ified and methyl-esterified pectin bridges were maintained throughout pollen de-
velopment although these connections seemed to be less strong in mature pollen
