In one aspect, a process for treating woody plant material is provided, the process involving contacting the plant material with a continuous flow of an aqueous ethanol solution at elevated temperature and pressure under conditions that promote extraction of ethanol-soluble lignin from the plant material and retention of hemicellulose sugars, xylose and cellulose in the treated plant material solids. In another aspect, a process for extracting hemicellulose sugars from lignin-depleted plant material solids is provided, the process involving contacting lignin-depleted plant material with water at elevated temperature and pressure under conditions that promote extraction of hemicellulose sugars from the plant material; and recovering hemicellulose sugars from the liquid mixture.