Cells of the medulla of Carnegiea gigantea may continue to grow for nearly a century and to be active until the death of the tree. Parenchymatous cells of the medulla retain their meristematic condition during the longer period mentioned. Great numbers of parenchymatous cells of all ages are transformed into vascular bundles forming strands connecting with the woody cylinder. The estimated rate of enlargement of medullary cells is greater in the 2nd than in the 1st half century following the initial distention. Growth occurs within periods of 150 days annually. Environmental conditions are favorable to activity except for a few periods of a few hours'' duration of low temperature each year. Pentosans or mucilages, abundant in young cells, diminish with age. Glucose content increases with age. Silica crystals become more abundant with age. Plasmatic layers shrink with age and become granular, while the nuclei change but little. Cell walls become thickened with age. The H-ion concentration varies only within narrow limits, p h 5.3-5.7, which is about that of a pectin or pentosan solution. Fatty substances are abundant in young cells in winter, their disappearance being followed by the appearance of starch in young cells. Immersion of young cells in acid of alkaline solutions increases permeability and diminishes water-holding capacity of young cells. Similar immersion of old cells in graded concentrations is followed by maximum swelling in acid solutions at p H 3-3.5 and a secondary maximum in alkali at p h 9-11. Such nodes have been attributed to the influence of isoelectric regions of proteins in Opuntia. The possible effects of variations in composition of the wall with age were not tested.