Q55. In spiral phyllotaxis, leaves are initiated sequentially on the meristem with
two successive primordia being separated by golden angle. If a plant
follows right–handed spiral phyllotaxis when looked down the meristem,
then the angle between two successive leaves would be _________degrees
(with correct sign, round off to one decimal place).
In spiral phyllotaxis, the golden angle separates successive leaf primordia on the meristem, calculated as 360° × (1 – 1/φ²), where φ ≈ 1.618034 is the golden ratio, yielding approximately 137.5078°. For a right-handed spiral viewed down the meristem (from apex toward base), this angle is taken as positive by convention in plant biology. Rounded to one decimal place, the angle between two successive leaves is 137.5°.
Detailed Solution
Leaf primordia emerge sequentially at the shoot apical meristem in a spiral pattern. The divergence angle is the shorter arc (less than 180°) between positions of successive primordia, optimized at the golden angle to minimize overlap and maximize light exposure. This value derives from φ = (1 + √5)/2, so 1/φ ≈ 0.618034 and 1/φ² ≈ 0.381966, thus 360° × 0.381966 ≈ 137.5078°. Right-handed means clockwise progression when looking down, assigned positive sign (+137.5°).
No Options Provided
The query mentions “explain every option,” but none are listed, typical for numerical fill-in CSIR NET questions. Common distractors might include 222.5° (360° – 137.5°, the supplement, incorrect as it exceeds 180°), -137.5° (wrong sign for right-handed), or 144° (Fibonacci approximation 360°/2.5, less precise).
In spiral phyllotaxis golden angle patterns, plants initiate leaf primordia sequentially at the shoot apical meristem, separated by the golden angle for optimal packing and sunlight capture. This angle, tied to the Fibonacci sequence and golden ratio (φ ≈ 1.618), equals 360° × (1 – 1/φ²) ≈ 137.5078°, rounded to 137.5 degrees. For right-handed spirals—clockwise when looking down the meristem—the angle carries a positive sign (+137.5°).
Why 137.5 Degrees?
The golden angle prevents alignment in straight files, forming parastichies (visible spirals) like 5/13 or 8/21 Fibonacci fractions on mature stems. Biophysical models show it minimizes energy for transitions between patterns during growth. Measured from older to younger primordia, it’s the smaller angle (<180°).
Right-Handed Convention
Viewed down (apex to base), right-handed spirals advance clockwise, denoted +137.5°; left-handed are counterclockwise (-137.5°). Both occur equally in nature (~50%), controlled by auxin transport and meristem geometry.
CSIR NET Relevance
This concept tests plant anatomy, developmental biology, and mathematical biology. Practice: Calculate via φ = (1 + √5)/2; divergence = 360°(1 – 1/φ²). Variants include distichous (180°) or decussate (90°/180°), but spiral dominates.
