In the third paper of this series (Bot. Mag. Tokyo, 1927), preliminary results of experiment done on the relation between the secretion of the stigma and the fertility in Petunia was reported. In this paper additional data obtained from further studies will be presented. 1. After washing off the secretion of the stigma with water an intra-self pollination was made, and then the results obtained are compared with those of the ordinary intra-self pollination. In the selfincompatible individuals the fertilizing percentage, the number of seeds, the size of either unfertilized overies or of fertilized capsules as well as the size, weight and germinating percentage of the seeds, the length of seedlings etc. were found to be larger in the case where the stigma was previousely washed than in the case where the ordinary intra-self pollination was carried on. But in the self-compatible ones the results were just the opposit of this. 2. The germination of the pollen was tested with two kinds of solutions: one containing the stigma-secretion of the same flower in addition to 1% grape sugar and the other nothing but the same amount of grape sugar. In the self-compatible individuals the germinating percentage was larger when the solution containing the secretion was used. But in the self-incompatible ones the results were just the opposit showing that the secretion of the stigma seems to inhibit the germination of the pollen of the same individual, and that higher the degree of selfincompatibility is, the more pronounced will this phenomenon become. 3. From the results of these two experiments, it may be said that the secretion from the stigma of a self-incompatible plant may inhibit the germination of its own pollen and that this may be, at least, one of the causes of self-incompatibility in Petunia. 4. The intra-self pollination was made after the washed stigma had been smeared with the secretion from that of a different individual of the same vegetative line, which is cultured under somewhat dry condition, or with that of another vegetative line cultured under the ordinary condition. And then compared their results with those of ordinary intra-self pollinations. 5. The germination of the pollen was tested with three kinds of sugar solutions: one with the stigma-secretion of the same flower, the other with that of a different individual of the same vegetative line under the dry condition, and still other with that of the other vegetative line. 6. The results of these two kinds of experiments showed that the secretion from the stigma of a different individual of the same vegetative line was more beneficial to both fertility and the germination of pollen, than that of the same flower, that of different vegetative lines being the most effective. These may be, at least, one of the causes of the facts that the fertilization of Petunia is more successful when the pollination was done between two individuals of the same vegetative line, cultivated under different condition, or those of the different line than self pollination was made.