The ultrastructure of ray and fusiform cells within the active vascular cambial zone of taproots of Aesculus hippocastanum L. is described. Both cell types are uninucleate and highly vacuolate and contain perinuclear and parietal populations of plastids and mitochondria, abundant rough endoplasmic reticulum, numerous dictyosomes and associated smooth vesicles, coated vesicles, numerous free ribosomes and polysomes, axially oriented microfilament bundles, and randomly oriented cortical microtubules, and they bear unlignified primary cell walls. Oleosomes, microbodies, and amyloplasts are more common within ray cells than in the fusiform cells and provide the main ultrastructural difference between the two. Generally, the ultrastructure of root cambial tissue is similar to the shoot cambium of this species and to the shoot cambia of other hardwood species. On the basis of ultrastructure, it has not been possible to identify true cambial initials in this tissue. Aspects of wall biosynthesis and chemistry wer...