Cucumber collapse

Before and After of the cucumber seedlings

Something mysterious happened to my cucumber seedlings. I originally sprouted them in the heated propagator, and I was so smug thinking "ha! look at my huge seedlings! Summer salads here I come!"... Clearly I spoke too soon. I had 6 decent-looking plants in no time, and then one by one they started collapsing - you can see the stages of decay in the photo. I am not sure what happened to them, but I have a theory: the heated propagator was not very sunny, and I wasn't quick enough moving the sprouted seed into the light. The plant grew too tall too soon, and the roots didn't have time to develop, so eventually they couldn't support the stem... I have sown some new seeds, but this time I'm trying a new approach and simply putting them in the windowsill next to the radiator. They have less heat, but at least the plant will grow slow but healthy.

El misterio de los pepinos:
Sembre unas semillas de pepino en el semillero electrico, y nacieron rapidamente. Las pase a la ventana, donde hay mas luz, y mirad que tallos tan esplendidos tenian! (La maceta de atras, en la foto). Pero de repente, una tras otra, las plantitas empezaron a "derrumbarse", y al final las seis que tenia acabaron por los suelos. Yo tengo una teoria: creo que nacieron demasiado rapido en el semillero con poca luz, y cuando las pase a la ventana ya tenian un tallo demasiado largo. Las raices no tuvieron tiempo de desarrollarse y la planta se vino abajo entera y murio. He vuelto a sembrar mas, pero esta vez voy a probar un metodo distinto: las he puesto junto a la ventana y el radiador, con menos calor pero mas luz, a ver si esta vez las plantas crecen despacio pero mas sanas.