The bacterium elongates and then divides in the middle to form 2 daughter cells.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
As the pileus elongates the stem does also, but more rapidly.
"Studies of American Fungi. Mushrooms, Edible, Poisonous, etc."
George Francis Atkinson
As the stem elongates more rapidly than the cap, the latter is lifted up away from the base of the stem.
"Studies of American Fungi. Mushrooms, Edible, Poisonous, etc."
George Francis Atkinson