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Show 136 GOOD FROM CROSSING. CHAP. XVII. given, but different, inasmuch as individual plants alone of the species are self-impotent. This self-impotence does not depend on the pollen or ovules being in a state unfit for fertilisation, for both have been found effective in union with other plants of the same or of a distinct specif:)s. The fact of these plants having spontaneously acquired so peculiar a constitution, that they can be fertilised more readily by the pollen of a distinct species than by their own, is remarkable. These abnormal cases, as well as the foregoing normal cases, in which certain orchids, for instance, can be much more easily fertilised by the pollen of a distinct species than by their own, are exactly the reverse of what occurs with all ordinary species. For in these latter the two sexual elements of the same individual plant are capable of freely acting on each other; but are so constituted that they are more or less impotent when brought into union with the sexual elements of a distinct species, and produce more or less sterile hybrids. It would appear that the pollen or ovules, or both, of the individual plants which are in this abnormal state, have been affected in some strange manner by the conditions to which they themselves or their parents have been exposed; but whilst thus rendered self-sterile, they have retained the capacity common to most species of partially fertilizing and being partially fertilized by allied forms. However this rna y be, the subject, to a certain extent, is related to our general conclusion that good is derived from the act of crossing. Gartner experimented on two plants of Lobelia fulgens, brought from separ~te places, and found 64 that their pollen was good, for he fertilised With It L. card~~alis and syphilitica ; their ovules were likewise good, for they were fer.tilised by the pollen of these same two species; but these two plants of L. julge1~s could not be fertilised by their own pollen, as ~n generally be effected mth perfect ease with this species. Again, the pollen of a plant of ~~r~ascum nigrum grown in a pot was found by Gartner r.s capable of fertilismg V. lychnitis and V. .Austriawrn · the ovules could be ~e~1ilised by the pollen of V. thapsus; but the fl;wers could not be fertilised by their own pollen. Koheuter, also,66 gives the case of three 64 'Bastarderzeugung,' s. 64, 357. 65 Idem 8 357 66 'Zweite Fortsetzung,' s. 10; 'Dritte Fort.,' s. 40. ' . . CHAP. XVII. SELF-IMPOTENT PLANTS. 137 garden plants of Verbascum phamiceum, which bore during two years many flowers ; these he successfully fertilised by the pollen of no less than four distinct species, but they produced not a seed with their own apparently good pollen; subsequently these same plants, and others raised from seed, assumed a strangely fluctuating condition, being temporarily sterile on the male or female side, or on both sides, and sometimes fertile on both sides; but two of the plants were perfectly fertile throughout the summer. It appears 67 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,68 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 69 found that he could not get fruit from P. alata and mcemosa except by reciprocally fertilising them with each other's pollen. Similar facts have been observed in Germany and France ;70 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. crerulea, and in another case with that of P. edulis. So again, with respect to P. laurifolia, a cultivator of much experience has recently remarked 71 that the flowers "must be fertilised with the pollen of P. crerulea, 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: 72 plants of Passijlora racemosa, cC£rulea, 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. cC£rulea, 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 sponta- 67 Duvernoy, quoted by Gartner, ' Bastarderzeugung,' s. 334. 6s ' Gardener's Chronicle,' 1846, p. 183. . 69 ' Transact. Hort. Soc.,' vol. vii., 1830, p. 95. 10 Prof, Lecoq, 'De la Fecondation,' 1845, p. 70; Gartner, ' Bastarderzeugung,' s. 64. 71 'Gardener's Chron.,' 1866, p. 1068. 72 'Journal of Proc. of Linn. Soc.,' vol. viii., 1864, p. 168. |