Whenever a number of individuals of the orchid Dendrobium crumenatum Lindl. occur within the same general locality the plants flower simultaneously. Such gregarious flowering is met with in other plants: for example, in certain bamboos such as Bcambusca arundincaceca (i), Phyllostcachys puberula (2), and Chusquea cabietifolica (3). There are, however, two striking differences between the simultaneous flowering of the orchid and that of bamboos. First, in the latter case the flowering is rhythmic. In bamboos which exhibit rhythmic sexual periodicity the time between periods of flowering, i.e., the life of the plant, is about thirty-two years (3, 4). In Dendrobium the periods between flowering dates vary from a few days to several months. A glance at the dates of flowering in table i is sufficient to reveal the fact that there is no rhythmic periodicity here. The second striking difference between the gregarious flowering of orchids and that of bamboos is that in the latter case all the individuals of a bamboo forest are of the same age, while among an assemblage of orchids the individuals may be of quite different ages. In the case of bamboos there is accordingly good reason to regard the sexual periodicity as the expression of an innate, heritable character (3). The lack of rhythm in the flowering of the orchid and the differences in the ages of the plants which flower gregariously suggest that simultaneity here is possibly due to an external factor. The following additional observation tends further to support this belief. Among the specimens of Dendrobium crumenatum in the Botanic Gardens at Buitenzorg, Java, there are plants collected from nearly all parts of the Dutch East Indies, from Riouw (near Singapore), from Sumatra, Java, Borneo, Celebes, and Ambon (a small island at the eastern end of the archipelago). These plants, shortly after being brought to Buitenzorg, all flowered on the same day, if they flowered at all; yet in their native habitats the flowering periods of the plants do not at all coincide. Thus, orchids growing in the virgin mountain forests flower on different days from those in the lowlands. Plants growing at two stations but three kilometers apart may differ in their times of flowering by one or two days. But wherever their original home and whatever the date of flowering there, the plants, when assembled in one locality, flower simultaneously with each other and with the plants which have grown in that locality from youth. Without