BSA Plant Images Online - Pollen Germination and Tube Growth in the Snow Buttercup, Ranunculus adoneus
Cross section through the dehisced anther of Xylopia collina of the pawpaw family (Annonaceae). Species of the genus Xylopia, as well as other members of this family, produce large pollen that is shed in units of four or sometimes even 32 grains (some tetrads visible in illustration). Associated with these compound pollen units in most genera are layers of sterile tissue, called septa, that separate the grains into chambers within each anther. Tsou and Johnson investigated the variation and development of the septa of Annonaceae, and found that despite variability in appearance, the septal tissues of all species were formed by the same developmental pathway. They propose that these tissues may have evolved in Annonaceae in response to a requirement for extra nutrients and support tissues for large pollen units.