The cluster roots of Grevillea robusta A. Cunn. ex R. Br. are composed of determinate rootlets that stop growing, but remain physiologically active for several months. Their apical organization, both before and after maturation, was studied by light and transmission electron microscopy. Each cell layer forms a dome, with an initial cell at its end. Xylem elements form a complicated triarch array at the base of the rootlet, passing along the rootlet as two files, and then joining at the tip to form a single file, surrounded by six pericycle cells. At the base of the rootlet, shorter xylem cells and thick-walled support cells are visible. A root cap, present in rootlets grown in vermiculite, was eventually displaced by root hair growth. Rootlets grown in Hoagland's solution lacked root caps and were significantly shorter than those grown in vermiculite. Cell fate was analysed in terms of cell position and is discussed in terms of pattern and development.

Light, fluorescence and electron microscopy were employed to follow the development of the endodermis in cluster roots and lateral roots of Grevillea robusta A. Cunn. ex R. Br. Endodermal cells had three different origins: rootlet endodermis arose from the rootlet meristem; endodermis covering the primordium shortly after initiation came from division of parental endodermis; cells at the junction between parent and rootlet endodermis developed from re-differentiated rootlet cortical cells. In the cluster root, the Casparian band formed in three ways, and was not initially present opposite the two sets of single xylem elements in the rootlet stele. A new clearing technique was developed that allowed visualization of xylem, suberized endodermis, Casparian band formation and phenolic compounds. In lateral roots, endodermal differentiation was asynchronous, but was related to position relative to protoxylem poles. However, the observed delay began before these poles had differentiated. At the tip of mature rootlets, which are determinate, the endodermis terminates in a ‘dome’ of cells, with the initial cell differentiating as an endodermal cell. Results are discussed in terms of determinate development in roots and the spatial and temporal contexts within which this development takes place.