It appears[311] that certain flowers on certain plants of _Lilium candidum_ can be fertilised more easily by pollen from a distinct individual than by their own. So, again, with the varieties of the potato. Tinzmann,[312] who made many trials with this plant, says that pollen from another variety sometimes "exerts a powerful influence, and I have found sorts of potatoes which would not bear seed from impregnation with the pollen of their own flowers, would bear it when impregnated with other pollen." It does not, however, appear to have been proved that the pollen which failed to act on the flower's own stigma was in itself good. In the genus Passiflora it has long been known that several species do not produce fruit, unless fertilised by pollen taken from distinct species: thus, Mr. Mowbray[313] found that he could not get fruit from _P. alata_ and _racemosa_ except by reciprocally fertilising them with each other's pollen. Similar facts have been observed in Germany and France;[314] and I have received two authentic accounts of _P. quadrangularis_, which never produced fruit with its own pollen, but would do so freely when fertilised in one case with the pollen of _P. coerulea_, and in another case with that of _P. edulis_. So again, with respect to _P. laurifolia_, a cultivator of much experience has recently remarked[315] that the flowers "must be fertilised with the pollen of _P. coerulea_, or of some other common kind, as their own pollen will not fertilise them." But the fullest details on this subject have been given by Mr. Scott:[316] plants of _Passiflora racemosa_, _coerulea_, and _alata_ flowered profusely during many years in the Botanic Gardens of Edinburgh, and, though repeatedly fertilised by Mr. Scott and by others with their own pollen, never produced any seed; yet this occurred at once with all three species when they were crossed together in various ways. But in the case of _P. coerulea_, three plants, two of which grew in the Botanic Gardens, were all rendered fertile, merely by impregnating the one with pollen of the other. The same result was attained in the same manner with _P. alata_, but only with one plant out of three. As so many self-sterile species have been mentioned, it may be stated that in the case of _P. gracilis_, which is an annual, the flowers are nearly as fertile with their own pollen as with that from a distinct plant; thus sixteen flowers {138} spontaneously self-fertilised produced fruit, each containing on an average 21.3 seed, whilst fruit from fourteen crossed flowers contained 24.1 seed.