The stamen and the pollen grain

A flower is where a flowering plant reproduces. It grows from the shoot tip, so it is really a modified shoot. Its parts sit in four rings, called whorls, from the outside in:

• Calyx: the green sepals, which protect the bud.
• Corolla: the coloured petals, which attract insects for pollination.
• Androecium: the male part, made of stamens.
• Gynoecium: the female part, made of one or more pistils, in the very centre.

The sepals and petals help only indirectly; the stamens and pistils are the reproductive parts.

A stamen has a long stalk, the filament, and at its tip an anther. A typical anther has two lobes (it is bilobed), and each lobe has two chambers (thecae), so it is called dithecous. Cut across, it shows four pollen sacs, the microsporangia, one in each corner.

Each microsporangium is wrapped in four wall layers, from the outside in:

• Epidermis: one layer that protects.
• Endothecium: helps the anther split open (dehisce) to release pollen.
• Middle layers: two or three layers that soon break down.
• Tapetum: the innermost layer. Its cells have dense cytoplasm and often more than one nucleus, and they nourish the developing pollen, like a mother feeding a baby.

The centre of each young microsporangium is filled with a tightly packed mass of similar cells, the sporogenous tissue. Each of these cells can become a pollen mother cell (microspore mother cell), which divides by meiosis into four cells together, a microspore tetrad. This process is microsporogenesis. As the anther matures and dries, the four cells separate and become pollen grains.

A pollen grain is the male gametophyte. Its wall has two layers:

• The hard outer exine is made of sporopollenin, one of the toughest natural substances known. It survives high temperatures, strong acids and alkalis, and no enzyme can break it down. That is why pollen is so well preserved as fossils. The exine has gaps, the germ pores, where the pollen tube will come out.
• The thin inner intine is made of cellulose and pectin.

Inside, a mature pollen grain has a large vegetative cell and a small generative cell. In over 60% of flowering plants, pollen is shed at this 2-celled stage; in the rest, the generative cell has already divided into two male gametes, so the grain is 3-celled.

Pollen matters to people too. It is rich in nutrients and sold as a food supplement, but some pollen causes allergies and asthma (the weed Parthenium is one source). Pollen can be stored for years in liquid nitrogen at –196 °C, in pollen banks, for use in plant breeding.