The length of epidermal cells in mature petioles of strawberry plants growing in different environments was measured and the ratio of petiole length to mean cell length calculated to give an estimate of the number of cells in a hypothetical linear column. The cells were both larger and more numerous in long photoperiods than in short, but the width of the cells was not affected. Gibberellic acid also increased both the length and number of cells, thus simulating the effect of long photoperiods. Extending the photoperiod by using incandescent lamps alone gave longer cells but no increase in number compared with using incandescent and fluorescent lamps together. Sampling a series of petioles, from the first, second and third leaves to emerge after the start of treatment, showed that the increase in petiole length in the first leaf to emerge following the application of gibberellic acid or the transference of the plants to continuous light or darkness, was almost entirely due to an increase in cell length. Further increments in petiole length in the second and third leaves were due entirely to increases in cell numbers, length in some cases declining.