Willeke,
The taller ones look to be reaching for light. I have been faced with this also and can be insufficient light from when first germinated or afterwards.
I most recently had a similar thing happen with a batch of seedlings. The seeds I received had very irratic germination. Adenium seedlings do not need strong light to germinate, but should receive 50% sunlight or strong artificial lighting as soon as they break soil, within a day to two.
I left these seeds in the propagator to retain humidity, as the first seeds germinated within 5 days and subsequent others over the next 8 days. The first became a bit etiolated from this, as I did not put under my lights until they where all up. Generally with good seed and relatively close germination rates, within days of each other.
Growing them under lights in our climate definitely assist for more even growth, but my feelings are that it is good light and more over high temps. which help most.
Your seedlings look healthy and green is good. Size of seedlings can vary so much even just with species, hybrid of seed and genetics. Not always in our control.
Rick
