Plants grown in darkness or deep shade stretch abnormally as they search for light. This causes elongated stems, small leaves, and a pale yellow or white color because the plant fails to produce chlorophyll. Growers often see this when seedlings are started too far from a grow light or when plants are left under opaque covers.
Heard on the job
The tomato seedlings in the back corner got completely etiolated, so we had to pinch them back and lower the lights.
Insider tip
Etiolated seedlings are notoriously fragile and prone to damping off; burying the elongated stems deeper during transplanting can sometimes save them.