what does the poet compare their branches to

In the poem "The Trees" by Adrienne Rich, the poet compares their branches to newly discharged patients who look half-dazed as they move towards the hospital doors after long illnesses and wait to get out of the hospital. The branches are described as "long-cramped" and have been shuffling under the roof, just like the patients who want to get out of the hospital and into the fresh air. The comparison is used to show that the branches also want to go out of the roof and desire to spread themselves in the fresh air, just like the patients who want to leave the hospital and recover in the outside world.