Shoot Apical Meristem - Theories of Organisation

Because a shoot tip has to do two jobs simultaneously - keep the main axis growing upward and keep budding off new leaf primordia sideways - botanists have argued for over a century about exactly how its cells are organised, and three named theories survive in the textbooks. The oldest, Hofmeister's Apical Cell theory (1852), keeps things simple: one single large apical cell at the very tip is the structural and functional unit from which everything else derives, an idea that fits algae, bryophytes and some pteridophytes reasonably well but struggles to explain the layered organisation seen in seed plants. Hanstein's Histogen theory (1868) replaces the single cell with three concentric zones, each destined for a different tissue: an outer Dermatogen that becomes the epidermis, a middle Periblem that becomes the cortex, and an inner Plerome that becomes the central stele. A. Schmidt's Tunica-Corpus theory (1924) simplifies this again into just two functional zones rather than three histogens: a peripheral Tunica, which divides mainly at right angles to the surface and forms the epidermis, and an inner Corpus, which divides in every plane and gives rise to the …