The stem or receptacle is hollow through the center, and this hollow opens out at the end so that there is a rounded perforation through the upper portion of the pileus.
"Studies of American Fungi. Mushrooms, Edible, Poisonous, etc."
George Francis Atkinson
It appears to be the case that most films on the surface of water show this tendency to perforation, which may be due to inequalities in the thickness of the film, or in the distribution of the strain to which it is subjected.
"Liquid Drops and Globules, their Formation and Movements"
Chas. R. Darling
Some of the larger globules have undergone perforation in several places, forming honeycombed plates.
"Liquid Drops and Globules, their Formation and Movements"
Chas. R. Darling