Maize tassel and anther development. Fertile maize anthers contain four locules. At ~0.1 mm there are about 50 cells in each locule. The line drawing of a transverse section (a) shows that the L2-derived (L2-d) cells are distinguishable from the L1-derived epidermis (ep) within a locule. At 0.3 mm (b) the L2-d has developed into endothecium (en), secondary parietal (2p) and archesporial cell types (ar). At 1.0 mm (c) middle layer (ml) and tapetum (tp) are apparent after a periclinal division of the 2p layer cells. Apart from the general morphology of no anther exertion, the mature tassel on the male-sterile am1-praI plant (e) is highly similar to a fertile sibling tassel (d) with the same number of tassel branches. In contrast, the am1-489 tassels (g) have fewer side branches (5-6) than fertile siblings (f). The individual reproductive units on the tassel are spikelets (sp). There are two florets in each spikelet, and each floret contains 3 anthers (h). The adaxial, upper florets used in the array experiment are developmentally 1 day ahead of the abaxial, lower florets. Anthers on a male sterile am1-praI plant (i) can reach 4-5 mm in length before they shrivel and degenerate; am1-489 anthers show growth arrest at 2-2.5 mm (data not shown). Based on cytological evidence and published results, anther sizes and the developmental timeline showing days after locular primordium initiation are diagrammed (j).