Sentence:  ... Thou / For whose path the Atlantic's level powers / Cleave themselves into chasms, while far below / The sea-blooms and the oozy woods which wear / The <WSD>sapless</WSD> foliage of the ocean, know / Thy voice, and suddenly grow grey with fear, / And tremble to despoil themselves: O, hear! 
Meaning of the word marked with <WSD> tags:  Lacking in sap.