Androecium

Androecium

The stamens are the pollen-bearing (male) organs. They typically consist of a stalk (the filament), and the pollen-producing part, the anther, which consists of four microsporangia separated into two pairs (thecae), linked by a connective. Each theca possesses two sporangia or anther locules divided by a septum.

Stamen filaments are typically slender and cylindrical, but in some species they are flattened and leaf-like (e.g. inNymphaea odorata) or even branched (e.g. in Ricinus communis). In many polyandrous angiosperms the stamens are borne in groups (fascicles). As in other floral parts, the filament surface often bears trichomes, stomata and surface patterning. In transverse section the filament possesses a parenchymatous ground tissue surround-ing the vascular tissue, which normally consists of a single vascular bundle.

The anther wall consists of several layers of cells. The epidermis normally undergoes only anticlinal divisions during development. The other anther wall layers are all derived from the primary parietal cells, which arise from the same initial cells as the primary sporogenous cells. The two most distinct anther wall layers are the endothecium, which lies immediately within the epidermis, and the tapetum, which is the innermost layer of cells surrounding the

anther locule (Fig. 5.9). Intervening layers usually consist of thin-walled cells that are often crushed and destroyed at anthesis. Endothecial cells typically develop fibrous wall thickenings which contribute to the anther dehiscence mechanism.

The tapetum is a specialized cell layer that functions as a source of nutrients for developing pollen grains. Tapetal cells are secretory and contain dense cytoplasm. They produce exine precursors, proteins and lipids that form the pollen coat. In many species a layer of tapetal cells remains intact around the anther locule; this type of tapetum is termed the secretory type (or cellular, glandular or parietal). In some other species the tapetal cells degenerate and their protoplasts fuse to form a multinucleate tapetal plasmodium (a periplasmodium) in the anther locule; this type of tapetum is termed the plasmodial type (or amoeboid, invasive, periplasmo-dial or syncytial type). Transitional types also occur in some species, especially in magnoliids.