Shoot unit and root shoot synchronism. a Definition of a shoot unit. The shoot unit (phytomer) is made up of a tiller bud, a leaf, and a node. Note the position of the upper and lower roots. b Schematic view of internal stem structure and the position of the embryonic roots. C = coleoptilar phytomer. c Synchronized emergence of leaves and crown roots. Arbitrary numbers were given to successive internodes of the stem.