in Birches the author uses the phrase shed crystal shells to describe

A collection of glass figurines
the ice cracking off of the trees
the melting of snow on the rocks
the shine of seashells glistening on a beach

the ice cracking off of the trees

In "Birches," the author uses the phrase "shed crystal shells" to describe the ice cracking off of the trees.

In the poem "Birches," the phrase "shed crystal shells" is used to describe the ice cracking off of the trees. This phrase symbolizes the delicate and fragile nature of the ice as it breaks away from the branches. To understand this, one needs to analyze the text.

To determine the correct answer, it is helpful to read the lines preceding and following the phrase "shed crystal shells." In "Birches," the relevant lines are:

"So low for long, they never right themselves:
You may see their trunks arching in the woods
Years afterwards, trailing their leaves on the ground
Like girls on hands and knees that throw their hair
Before them over their heads to dry in the sun."

These lines mention the bending and arching of the birch trees, implying that the ice, which earlier weighed them down, has cracked and fallen off. The phrase "shed crystal shells" can be interpreted as the ice breaking away from the trees, resembling shells that have been discarded or shed. This description aligns with the fragility and transparency of the ice.

By analyzing the context and imagery in the poem, one can identify that the phrase "shed crystal shells" refers to the ice cracking off of the trees.