That the pollen grains of various plants might be induced to germinate under artificial conditions has been known for a long time. Several investigators have adopted this mode of attack in seeking to determine the requirements for pollen-tube growth. Extending the earlier findings of BobilioffPreisser (2) and Knight (5) with Vinca minor and the apple respectively, the present writer (Brink, 3) has shown that pollen tubes of Scilla, Chionodoxa, Puschkinia, Muscari, and Vinca equal in length to those growing in the pistils of these forms may be secured on a culture medium consisting of cane sugar, sterile yeast, and agar. It was demonstrated also that, in the Scilla tubes grown under artificial conditions, the nuclei were in position to function had fertilization been possible. These results show that pollen-tube growth, constituting as it does an essential link in the process of sexual reproduction among the higher plants, may be imitated in vitro, at least in regard to the prime factors of length of tube and nuclear distribution. They lend support to the view, moreover, that this method of approach may ultimately furnish the facts necessary to an interpretation of the physiological processes involved and throw light upon certain related phenomena of primary interest in plant genetics. From the results of the studies noted above and others, various helpful conclusions have been drawn. The methods employed, however, left much to be desired from the quantitative standpoint. With the media used, an adequate knowledge of the conditions of the experiments was often impossible. These experiments served, however, for the discovery of some of the more important variables and made it clear that further progress was dependent upon the development of a methodology that would permit of their effective control. Only under such conditions could a precise measurement of the various factors contributing to growth be made. The method described here for determining the effect of hydrogen-ion concentration on the development of pollen tubes is based upon the results of experiments on the influence of neutral salts. While the writer hopes to publish these results shortly, brief statement of some of the findings must be made at this time, since it was only through a knowledge of the effects of 1 Papers fromi the Department of Genetics, Agricultural Experiment Station, University of Wisconsin, no. 4I. Published with the approval of the Director of the Station.