1 All types of roots of Sagittaria latifolia have the same histogenic regions: calyptrogen, dermatogen and periblem in common, and plerome. 2. The rootlets arising from the adventitious root probably come from a plate of pericyclic cells, since single origins show little evidence of anticlinal division adding to the lateral bulk of the meristematic group. 3. The plerome of the rootlet is probably differentiated with the first anticlinal division, but some sections indicate its later separation from the common meristematic group. The plerome arises as a plate of cells with the central cell more developed, and not from a single cell. 4. The inner layer of the mother root completely covers the tip of the rootlet and adds to its lateral bulk, but does not give rise to the calyptrogen. 5. The intercellular spaces of the cortex are of two kinds, gas spaces and latex canals. The gas spaces enlarge and break down most of the cortical cells. These spaces are interrupted by plates of live cells at right angles to t...