Winter's bark pollen. Coloured scanning electron micrograph of winter's bark Drimys winteri pollen. The pollen is found in associations of 4 grains, known as tetrads. The outer wall (exine) of the grain comprises rod- like processes (bacula), fused at their tips, forming a network of ridges (muri) and surrounding a circular germinal pore. When the pollen, containing male gametes, lands on the stigma of a flower, it germinates. A pollen tube grows out of a pore, down the stigma, to the ovary. Male nuclei pass down the tube and fertilize the ovules to form seeds. Winter's bark is a flowering shrub native to South America. Magnification: x1380 when printed 10cm high.

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