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We are talking here about leaves with lengthwise stripes of yellow or white tissue.

The green color in a normal, healthy leaf comes from the chloroplasts in its cells. The chloroplasts carry out the photosynthesis for the plant. Normal chloroplasts are green, because they contain functional chlorophyll. Some plants contain defective chloroplasts, which lack functional chlorophyll molecules and which therefore lack the green color. A variegated plant contains both of these types of chloroplasts. An albino plant contains only defective chloroplasts.

Whether a given leaf cell in a variegated plant shows the typical green color of normal, healthy tissue, or shows no green color, depends on whether it gets some normal, healthy green chloroplasts or gets only the defective, yellow or colorless, chloroplasts.

When a leaf cell divides into two daughter cells, its chloroplasts are distributed randomly between the two daughter cells. If it contains both normal and also defective chloroplasts, its daughter cells may get normal, or get defective, or get some of both types. The growing point for a new leaf is made up of many cells from all three layers of the meristem. The cells that contain the active chloroplasts are in one layer.

Note that the non-green tissues in a normal plant contain chloroplasts, but their chlorophyll function is turned off unless they are in the photosynthetic layer of a leaf. They are not defective, just turned off. Such tissues usually include roots, leaf bases, and flower petals.

The yellow or colorless cells in this type of variegated leaf are carrying defective chloroplasts, not just "turned off" chloroplasts.

The Bottom Line:

The unstable nature of most lengthwise striped variegation is due to the randomness in the distribution of chloroplasts between the daughter cells when a meristem mother cell divides into two cells.

Side Note:

"Akebono" variegation is a different phenomenon, where (probably) most or all chloroplasts can potentially have green functional chlorophyll, but under some conditions it is turned off.

Unstable variegation:

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