Part 4 explains Fibonacci numbers in greater detail but also introduces Lucas numbers and phi. 

Part 5 describes phi and its relation to the golden ratio and golden spiral.

Part 6 explores concentric circles and tessellations in nature.

Part 7 investigates hexagons and why and where they appear in nature.

Part 8 looks at fractals and meanders in nature.

Part 3 delves into Fibonacci numbers and where they appear in nature.