Read the stanzas from "Night."
 under at an unfaltering pace, 
 under till the rinds break, 
 back till each bent leaf 
 is parted from its stalk; 
 under at a grave pace, 
 under till the leaves 
 are bent back 
 till they drop upon earth, 
 back till they are all broken
 Why does the poet repeat the word "under" in these stanzas?