Isolated plants of _Linaria vulgaris_ do not produce seed, even if freely pollinated by bees. Pollen from other plants is required. This requirement is not at all restricted to the genus _Linaria_, as many instances are known to occur in different families. It is generally assumed that the pollen of any other individual of the same species is capable of producing fertilization, although it is to be said that a critical examination has been made in but few instances. This, however, is not the case, at least not in the present instance. I have pollinated a number of plants, grown from seed of the same strain and combined them in pairs, and excluded the visits of insects, and pollen other than that of the plant itself and that of the specimen with which it was paired. The result was that some pairs were fertile and others barren. Counting these two groups of pairs, I found them nearly equal in number, indicating thereby that for any given individual the pollen of half of the others is potent, but that of the other half impotent. From these facts we may conclude the presence of a curious case of dimorphy, analogous to that proposed for the primroses, but without visible differentiating marks in the flowers. At least such opposite characters [472] have as yet not been ascertained in the case of our toad-flax. In order to save seed from isolated plants it is necessary, for this reason, to have at least two individuals, and these must belong to the two physiologically different types. Now in the year 1892, as in other years, my plants, though separated at the outset by distances of about 20 cm. from each other, threw out roots of far greater length, growing in such a way as to abolish the strict isolation of the individuals. Any plot may produce several stems from such roots, and it is manifestly impossible to decide whether they all belong to one original plant or to the mixed roots of several individuals. No other strains were grown on the same bed with my plants however, and so I considered all the stems of the little group as belonging to one plant. But their perfect fertility showed, according to the experience described, that there must have been at least two specimens mingled together. Returning now to the seeds of this pair of plants, I had, of course, not the least occasion to ascribe to it any higher value than the harvest of former years. The consequence was that I had no reason to make large sowings, and grew only enough young plants to have about 50 in bloom in the summer of 1894. Among [473] these, stray peloric flowers were observed in somewhat larger number than in the previous generations, 11 plants bearing one or two, or even three such abnormalities. This however, could not be considered as a real advance, since such plants may occur in varying, though ordinarily small numbers in every generation.