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The rhizome of nine species of the genus Xyris L. (Xyridaceae) was analyzed in order to present anatomical evidence that the primary thickening of the stem is a result of meristematic activity in both the endodermis and pericycle. The activity of these two tissues was verified by demonstrating that the pericycle gives rise to vascular tissues centripetally and that cell divisions take place in the endodermis and lead to the formation of the cortex centrifugally; both processes lead to the primary thickening of the rhizome. The existence of two tissues with meristematic activity instead of a single meristem (the primary thickening meristem) was shown from anatomical characteristics such as the arrangement of radial rows of cortical cells derived from the endodermis, the lignification of the endodermis and its continuity with both the adventitious roots and in the leaf traces in the later stages of development, and the noncorrespondence and the difference in size between the cells of the two tissues responsible for thickening primary in the younger stages.