Q34 - Two-Preceding-Leaf Rule for Leaf Positioning in the Fibonacci Spiral

Theoretical 2 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Tropisms & Nastic Movements Plant Hormones

Plant leaves are arranged around the stem in regular patterns. A typical example is the Fibonacci spiral, where the angle between successive leaves is near the golden angle of 137.5 degrees. It is considered that the position of new leaf formation is determined under the epidermis-transmitted inhibitory effect from existing leaves. Researchers made a shallow cut on the surface at the adaxial side of the incipient leaf P0 to isolate it from the apical region, and measured the deviation from 137.5 degrees of the angles between successive leaves after the next leaf P1 and second-next leaf P2 formed. The deviation was small for most angles except angle P-1OP2, which showed a large (about 40 degree) deviation from 137.5 degrees.

Schematic drawing of the microsurgical Schematic drawing of the microsurgical

Deviation of the angle between experiment successive leaves from 137. Deviation of the angle between experiment successive leaves from 137.

Using the information and data, determine which of the statements are true or which are false.

conclusions. What can be speculated from the above experiment? Choose the most appropriate set of reasonable speculations: A. Among existing leaves, only the immediately preceding leaf is critical for the determination of the position of a new leaf formation. B. Among existing leaves, two preceding leaves are critical for the determination of the position of a new leaf formation. C. When a leaf is newly arising, the position of the next leaf is not determined yet. D. When a leaf is newly arising, the position of the next leaf is already determined but the position of the second next leaf is not determined yet. E. If n is sufficiently large, the deviation of angle PnOPn+1 from 137.5 degrees is close to 0. F. If n is sufficiently large, the deviation of angle PnOPn+1 from 137.5 degrees is close to 42.5 degrees.

← Previous: Q33 Next: Q35 →

Question reproduced from IBO 2020, Theoretical Exam 2, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)