The quiescent centres of angiosperm roots of a wide range of size and architecture have been investigated by autoradiography and stathmokinetic measurements. The volume and number of constituent cells of the quiescent centre vary with the width of the root, but roots of the Gramineae and Cyperaceae have larger quiescent centres for their size than all other plants except Pistia. The difference is not correlated with the open or closed nature of the meristem nor whether the epidermis is derived from the cortex or cap, but is correlated with a thickened boundary between the cap and the rest of the apex or, in Pistia, ith a discrete epidermal meristem. The number of cells in the quiescent centre approaches zero in the non-grass type of apex as the width of the root becomes smaller and nearer that of pteridophyte roots with tetrahedral apical cells. The ratio of rates of cell cycling in the quiescent centre to those in the surrounding meristem also varies with root width, but here all closed meristems show greater differences in rates than all open meristems. The ratio comes near to unity in the smallest open meristems. The data are discussed in relation to the reasons for quiescence, the differences between root and shoot meristems and our ability to perform certain surgical operations.