The Stamen, Microsporangium and Pollen Grain

The androecium is the whorl in which the male line of reproduction begins, and its functional

unit, the anther, is where the whole story of male gametophyte development plays out. A stamen

consists of a slender filament and, at its tip, a fertile anther, usually bilobed, with the two

lobes joined by a connective tissue through which the vascular strand supplying the anther runs.

Each anther lobe is dithecous, meaning it is itself divided into two chambers, so that a

typical bilobed anther contains four such chambers in total, called microsporangia (or pollen

sacs), one situated in each corner when the anther is viewed in transverse section. As the

microsporangium develops, its wall differentiates into four distinct layers, arranged from the

outside inward. The outermost is the epidermis, a single protective layer covering the anther.

Beneath it lies the endothecium, which plays an important mechanical role: as the anther matures,

its cells develop characteristic fibrous, radial thickenings that help the anther split open

(dehisce) at the right time to release its pollen. Within the endothecium lie two or three middle

layers, which are typically transient nutritive layers that are absorbed as the anther matures. The

innermost wall layer is the tapetum, the most biologically active of the four: its cells are

densely cytoplasmic and rich in food reserves, and it directly nourishes the developing pollen

grains within the microsporangium, being progressively consumed and absorbed as those pollen

grains mature.

Enclosed within each young microsporangium, surrounded by this four-layered wall, is a compact

mass of sporogenous tissue. In the fully differentiated anther, the cells of the sporogenous tissue

become microspore mother cells (also called pollen mother cells, PMCs) -- diploid cells that are

the direct precursors of pollen. It is this sporogenous tissue, and specifically its PMCs, that

undergoes the meiotic division described in the next section, converting diploid tissue into

haploid pollen.

Sporopollenin -- the material forming the outer wall (exine) of the pollen grains that will develop

from this tissue -- deserves special mention here because of just how unusual its properties are.

It is considered one of the most chemically and physically resistant organic substances known: no

enzyme capable of breaking it down has been identified, and it withstands treatment by strong acids

and alkalis as well as very high temperatures without significant degradation. This extraordinary

resistance means that pollen exine, once formed, can persist essentially unaltered for extremely

long periods, including within geological sediments long after the rest of the plant has decayed.

This durability underlies the science of palynology, in which fossil pollen assemblages -- each

species having its own distinctively ornamented exine -- are used to reconstruct past vegetation,

climate history, and to help date sedimentary deposits; pollen analysis is also used in forensic

science and in the study of pollen allergies.

Speaking of allergies: because wind-pollinated species release enormous quantities of small, light

pollen directly into the air (as opposed to insect-pollinated species, whose comparatively heavier, …