Question: Explain Variegation in Ivy (xvi): leaf initiation

My post on may 27th discussed the SAM and how it contributes cells to a leaf. Then, I didn't get into detail on how a leaf initiates from the SAM. However, to be able to explain the irregular pattern of streaked variegation you first need to understand leaf initiation.

All three layers of the SAM are involved in the following process. At regular intervals an increased rate of cell division occurs on the flank of the meristem. The developing bulge is called a leaf buttress. A leaf primordium is generated by local apical growth in the leaf buttress. At some point during apical leaf growth the leaf development begins.

Imagine the SAM as a dome-shaped structure. Imagine the population of mutated (albino) cells in the SAM as a 'moving' and 'changing' stain in that structure. When leaf initiation occurs on the flank of the structure the stain could be involved. Perhaps 1) the initiation occurs in the centre of the stain. Or 2) the initiation occurs only partly in the stain. However, it's also possible 3) the initiation doesn't occurs at all near the stain.

In situation 1) and 2) it's impossible to predict the outcome. The structure of the leaf primordium depends on the proliferation of the cells involved. The leaf colour could be yellow or the leaf pattern could be streaked. In situation 3) it's clear the colour of the leaf will be green (without the occurrence of another mutation).

Next: trying to predict a speckled pattern.