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From the picture below, it seems that new leaves are formed something like this (if you imagine the plant as a regular, 12 hour clock):

new leaf at 12 o'clock, then at 7 o'clock, then at 2 o'clock, then at 9 o'clock, and so on.

I imagine it could be some things as simple as 'always add 190 degrees'. Are the exact rules for this documented somewhere for various plants?

houseleek

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Spiral Phyllotaxis and the Golden Angle

The disposition of the leaves in your picture is not as simple as it seems. That's a spiral phyllotaxis, which can be very complicated.

The plant in your image is a houseleek, a plant from the Genus Sempervivum. In those plants, according to Jeremy Burgess (Introduction to plant cell developmnent):

... leaf primordia are separated from each other in an angular arc that approaches 137°.

By the way, 137° is (almost) the golden angle (φ).

Here is an image explaining the phyllotaxis of Sempervivum (from Burgess, 1989):

enter image description here

Even more interesting is the pattern made by the 137.5° GA spiral of Aloe polyphylla:

enter image description here

Sometimes this can produce invisible spiral patterns, a phenomenon called parastichy:

enter image description here

Source: Burgess, J. (1989). An introduction to plant cell development. 1st ed. Cambridge [etc.]: Cambridge University Press.

PS: I know that this is off-topic, but I'd like to share this video from Cristóbal Vila, which deals with Fibonnaci sequence, golden ratio and golden angle in nature. Look at the flower disposition at 1:44 : https://www.youtube.com/watch?v=kkGeOWYOFoA

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  • $\begingroup$ excellent, thanks! also thanks for the video link! $\endgroup$
    – Hoff
    May 8, 2017 at 13:30

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