Figure 1

The schematic illustration (left) and corresponding photograph (right) show the developmental stages of the sporangiophore of P. blakesleeanus. Sporangiophore development is divided into five stages (stages I, II, III, IV and V), and stage IV is further divided into three sub-stages (IVa, IVb and IVc). The sporangiophores exhibit elongation and spherical growth (volumetric or expansive growth) in a region termed the “growth zone” which is colored light green in the schematic illustration. The non-growing stalk is colored dark green. The growth zone is located at the apical tip of stage I (tip growth) and stage II (spherical growth) and adjacent to the sporangium (intercalary growth) in stage IV. The stage I sporangiophore is a pointed cell that grows longitudinally at the apical tip in the growth zone that is 1–2 mm in length. Clockwise rotation (when viewed from above) and elongation growth occur concurrently during stage I, producing left-handed helical growth. Stage II begins with spherical growth at the apical tip and does not exhibit elongation or rotation growth. Spherical growth continues until it a sporangium is formed (~ 0.5 mm in diameter) and then it stops. This is stage III, a period of spore formation inside the sporangium. During stage III, there is no visible expansive growth. Stage IVa begins with elongation growth concurrent with counter-clockwise rotation growth (right-handed helical growth) in a short growth zone located approximately 0.6 mm below the sporangium. The length of the growth zone, elongation growth rate, and rotation growth rate increase in magnitude as the right-handed helical growth continues for approximately one hour. Then, the rotation rate gradually decreases to zero, and clockwise rotation begins without interruption of elongation growth. Stage IVb begins with the initiation of left-handed helical growth. Stage IVb sporangiophores exhibit nearly constant growth zone lengths (~ 2.5 mm), elongation growth rates (~ 35 μm/min) and rotation growth rates (~ 12 degrees/min) for many hours. Stage IVc is initiated by counter-clockwise rotation and right-handed helical growth. Stage V does not exhibit visible expansive growth. These large cylindrical single-celled sporangiophores are approximately 150 μm in diameter and can grow in length to ten or more centimeters. [Schematic and photograph are taken from Ortega et al.⁵ and reproduced here with permission].