I think a little pressure helps to maintain good media contact and to keep tiny little air pockets from forming next to the scale itself--it sort of simulates the 'real world condition' of a little ground on top.
Here's a little update on my five year old Grandaughters' experiment in cloning I originally posted in 'chickensandmore's thread'. The scales were put into incubation in mid October, inspected on Nov. 20th.(pictures 1 and 2). Then returned to incubation until the Christmas Holidays. She wanted to plant two scales at that point (without vernalization). The remaing two scales were then vernalized at 34F until the first week of March, then planted.
Picture 3 shows her success from four scales, two vernalized and two grown straight thru. The vernalized clones are on the left in picture 3, The unvernalized clones are in their original right side as planted spot, undisturbed. On April 5th, this pot was moved outdoors to her little 'seed hothouse' (for pansies, petunias,etc.)and within days, the four clones from one of the unvernalized scales seemed to have been tricked into Fall dormancy (see yellow/tan leaf from the last remaining one). In picture 4, the vernalized ones are on the right.
These clones are No ID: T X T or T X A and nothing special, just a teaching excersize--but fun. Maybe I can-?-Nauh, a five year old--I've got to keep it simple.