"Wood does not occur in any plants of a lower grade than ferns; and in the higher plants in which it does occur it is chiefly, but not exclusively, in the stem. The main physiological function of wood is the as it grows erect and branches. Submerged s, buoyed up, as they are, by the water, do not form wood in their stems, nor, as a rule, do s, nor, at first, the succulent, flexible shoots of longer-lived plants. In , and in allied plants, the wood, though dense, consists largely of scattered longitudinal strands and often of cells of no great vertical length. Though there are also generally woody layers just below the surface of the stem, giving it considerable strength as a whole, the structure renders s useless as . For all practical purposes, therefore, wood is produced only by the highest sub-kingdom of the plant world, the seed-bearing or flowering plants, the or Phanerogámia of botanists."