When we take plants out of their normal substrate and wash their roots, or take cuttings, they go right into shock and start a kind of survival program. The more material they have to sustain, the more energy it will take, and roots are a very precious part for the plant so it will try to save them. When plants/cuttings have light, they can produce energy (in the form of sugar) in their leaves by photosynthesis, which will be transported down to the roots for feeding them. An uprooted plant will survive if it finds itself somewhere on the soil where it can grow new roots into. When plants are in a box with no light, however, they will "suck" the energy from the leaves which will thereby be harmed. For traveling in darkness it is better to not have roots, so the unrooted cutting remains in a state of shock; it will be dormant and the leaves sustain themselves for quite some time, so when times become better (i.e. the cuttings arrive in loving hands that pot them up and give them light and humidity) they still have enough energy to put into the formation of new roots. Without enough humidity in the surrounding air ( that can be provided by a terrarium or a large plastic bag ) plants or cuts will shed their leaves in the hours that follow unpacking.