1)Experimental mitosis stimulation in the differentiated root tissues in some species of Monocotyledons was obtained by treatment with some concentrations (generally, from 10 to 50 p. p. m.) of sodium 2,4-dichlorophenoxyacetate.2)In Tables I and II the percentages of polyploid mitoses in the cortex tissue in some of the species investigated herein are reported. In addition, a survey of the distribution of polyploidy in the various tissue layers in the roots of the species examined is given in the text.3)Polyploid cells showed 4nand 8nmitoses with « pairs » and « groups of four » chromosomes respectively. Only in one root inBellevalia romanapolyploid mitoses with single chromosomes, indicative of a second mitotic cycle in differentiated cells, were observed.A detailed description of the course of both 4nand 8nmitoses in differentiated root cells in the species studied is reported. The greatest variability in the behaviour of both centromere and relational coiling in 4nmitoses was observed in the roots ofAllium yunnanense.Spontaneous chromosome aberrations were met with in differentiated cells in bothAllium yunnanenseandNothoscordum fragrans.4)InGaltonia candicansandBulbinella Hookeri, notwithstanding the very intense pericyde stimulation for the production of lateral roots, mitosis stimulation in adult tissues was practically absent.5)The present observations are discussed in connection with our present knowledge on endopolyploidy and tissue differentiation and function. It is assumed that tetraploidy and octoploidy in differentiated tissues are the result of. respectively, a double and a triple chromosome reproduction in the resting stage, giving rise to « diplo » and « quadruplochromosomes ». The reasons for preferring the term « endopolyploidy » to « polyteny » are presented, with reference to a possible occurrence of an « endomitosis in a despiralized stage », as suggested by Geitler(1948).6)The following other points are also discussed:a)the chimerical constitution of differentiated plant tissues, 2n, 4n, 8nand probably higher polyploid nuclei occurring in different proportion in different tissues and in one and the same tissue in different species (or individuals in one species);b)the possible connection between size and shape of nuclei in adult tissues on one side and degree of « ploidy » on the other;c)the occurrence of spontaneous chromosome aberrations in differentiated cells and the possible significance of metabolites or some of their derivatives in the origin of the aberrations (cf. D'AMATO 1950b).