Close enough?

The majority of sunflower heads have 34, 55 or 89 seed spirals in one direction, all of which are Fibonacci numbers (0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89…). Almost 20 per cent, though, have something in between, with some matching numbers from series starting with different numbers, such as Lucas (2, 1, 3, 4, 7, 11,

18, 29, 47, 76, 123…), named after Édouard Lucas, who coined the name Fibonacci numbers, F4 (3, 1, 4, 5, 9, 14, 23, 37,

60, 97…) and double Fibonacci

(0, 2, 2, 4, 6, 10, 16, 26, 42, 68…).

A few other variations have been noted, such as a Fibonacci number plus or minus one, but that stretches the analogy too far.

It is sometimes stated that the number of petals in a flower or flower head is also a Fibonacci number, again citing sunflower as an exemplar. Yet, in a study of 1000 sunflower flower heads, just over a third had 21 and the rest anything between 12 and 31.