At the moment when the Osmia disgorges her honey and especially at the moment when, with her hind-tarsi, she rubs the pollen-dust from her ventral brush, she needs a narrow aperture, just big enough to allow of her passage.

I imagine that in a straitened gallery the rubbing of her whole body against the sides gives the harvester a support for her brushing-work.
In a spacious cylinder this support fails her; and the Osmia starts with creating one for herself, which she does by narrowing the channel.
Whether it be to facilitate the storing of the victuals or for any other reason, the fact remains that the Osmia housed in a wide tube begins with the partitioning.
Her division is made by a dab of clay placed at right angles to the axis of the cylinder, at a distance from the bottom determined by the ordinary length of a cell.

The wad is not a complete round; it is more crescent-shaped, leaving a circular space between it and one side of the tube.

Fresh layers are swiftly added to the dab of clay; and soon the tube is divided by a partition which has a circular opening at the side of it, a sort of dog-hole through which the Osmia will proceed to knead the Bee-bread.

When the victualling is finished and the egg laid upon the heap, the whole is closed and the filled-up partition becomes the bottom of the next cell.