Avers, CharlotteJ., and EarlE. King. (Douglas College, Rutgers U., New Brunswick, N. J.)Histochemical evidence of intracellular enzymatic heterogeneity of plant mitochondria.Amer. Jour. Bot. 47(3) : 220—225. Illus. 1960.–Mitochondrial counts were made of root tip meristem cells of 4 grass species after 4 different treatments, all without fixation, of whole roots. The mean numbers of mitochondria per cell stained with Janus green B were 85—95, and the mean numbers of mitochondria per cell showing cytochrome oxidase activity after the Nadi reaction were 90—95. There was no significant difference between these 2 sets of values for any of the species tested. Counts of cytoplasmic particles active for acid phosphatase showed mean values of 98‐108 per cell, which was highly significant in each species when compared with the Janus‐green counts. The succinic dehydrogenase system localized with Neotetrazolium showed mean numbers of mitochondria per cell of 42—58, which represented 50—60% of the Janus green‐stained mitochondrial populations and were significantly different. The evidence indicated the occurrence of enzymatic variation among mitochondrial complements of individual cells, as well as the existence of at least 2 kinds of cytoplasmic particles. The excess numbers of phosphatase‐active particles were considered possibly to be due to the activity of this hydrolase in lysosomes as well as in mitochondria. Thus, the evidence suggested the occurrence of both heterogeneity and multiplicity of cytoplasmic particles in relation to their enzymatic activities.