The root structure in members of the Lemnaceae is important to plant researchers, because changes during cell differentiation can more easily be monitored in short roots with determinate growth. Here, the structural organization and cellular differentiation of the root system was assessed in the highly reduced Spirodela polyrhiza. While protected by a prophyllous sheath, rapid cell division occurred in the apical and vascular regions of the immature roots. Concentric rings of endodermis with Casparian strips, cortex, and epidermis enclosed a single vascular strand. The cytoplasmic density of the cortex was high at the apex, but decreased progressively along the root. The root root cap junction, closely attached at initiation, later became a distinct boundary layer filled with fibrillar materials. Chloroplasts were well distributed. Numerous plasmodesmata indicated the likely symplastic movement of ions and metabolites in the root system as well as further into the reduced plant body. A high cytoplasmic density at the apex and extreme vacuolization along the cortex provided possible explanations for the considerable distribution of weight along the roots of the plant body. These conditions probably enable the root tip to serve as a pendulum against water motion.