In the seventeenth chapter of my ‘Variation of Animals and Plants under Domestication’ I had occasion to enter fully on the present subject; and I will therefore here give only a brief abstract of the cases there described, but others must be added, as they have an important bearing on the present work. Kolreuter long ago described plants of Verbascum phoeniceum which during two years were sterile with their own pollen, but were easily fertilised by that of four other species; these plants however afterwards became more or less self-fertile in a strangely fluctuating manner. Mr. Scott also found that this species, as well as two of its varieties, were self-sterile, as did Gartner in the case of Verbascum nigrum. So it was, according to this latter author, with two plants of Lobelia fulgens, though the pollen and ovules of both were in an efficient state in relation to other species. Five species of Passiflora and certain individuals of a sixth species have been found sterile with their own pollen; but slight changes in their conditions, such as being grafted on another stock or a change of temperature, rendered them self-fertile. Flowers on a completely self-impotent plant of Passiflora alata fertilised with pollen from its own self-impotent seedlings were quite fertile. Mr. Scott, and afterwards Mr. Munro, found that some species of Oncidium and of Maxillaria cultivated in a hothouse in Edinburgh were quite sterile with their own pollen; and Fritz Muller found this to be the case with a large number of Orchidaceous genera growing in their native home of South Brazil. (9/2. ‘Botanische Zeitung’ 1868 page 114.) He also discovered that the pollen-masses of some orchids acted on their own stigmas like a poison; and it appears that Gartner formerly observed indications of this extraordinary fact in the case of some other plants.
Fritz Muller also states that a species of Bignonia and Tabernaemontana echinata are both sterile with their own pollen in their native country of Brazil. (9/3. Ibid 1868 page 626 and 1870 page 274.) Several Amaryllidaceous and Liliaceous plants are in the same predicament. Hildebrand observed with care Corydalis cava, and found it completely self-sterile (9/4. ‘Report of the International Horticultural Congress’ 1866.); but according to Caspary a few self-fertilised seeds are occasionally produced: Corydalis halleri is only slightly self-sterile, and C. intermedia not at all so. (9/5. ‘Botanische Zeitung’ June 27, 1873.) In another Fumariaceous genus, Hypecoum, Hildebrand observed that H. grandiflorum was highly self-sterile, whilst H. procumbens was fairly self-fertile. (9/6. ‘Jahrb. fur wiss. Botanik’ B. 7 page 464.) Thunbergia alata kept by me in a warm greenhouse was self-sterile early in the season, but at a later period produced many spontaneously self-fertilised fruits. So it was with Papaver vagum: another species, P. alpinum, was found by Professor H. Hoffmann to be quite self-sterile excepting on one occasion (9/7. ‘Zur Speciesfrage’ 1875 page 47.); whilst P. somniferum has been with me always completely self-sterile.
Eschscholtzia californica.
This species deserves a fuller consideration. A plant cultivated by Fritz Muller in South Brazil happened to flower a month before any of the others, and it did not produce a single capsule. This led him to make further observations during the next six generations, and he found that all his plants were completely sterile, unless they were crossed by insects or were artificially fertilised with pollen from a distinct plant, in which case they were completely fertile. (9/8. ‘Botanische Zeitung’ 1868 page 115 and 1869 page 223.) I was much surprised at this fact, as I had found that English plants, when covered by a net, set a considerable number of capsules; and that these contained seeds by weight, compared with those on plants intercrossed by the bees, as 71 to 100. Professor Hildebrand, however, found this species much more self-sterile in Germany than it was with me in England, for the capsules produced by self-fertilised flowers, compared with those from intercrossed flowers, contained seeds in the ratio of only 11 to 100. At my request Fritz Muller sent me from Brazil seeds of his self-sterile plants, from which I raised seedlings. Two of these were covered with a net, and one produced spontaneously only a single capsule containing no good seeds, but yet, when artificially fertilised with its own pollen, produced a few capsules. The other plant produced spontaneously under the net eight capsules, one of which contained no less than thirty seeds, and on an average about ten seeds per capsule. Eight flowers on these two plants were artificially self-fertilised, and produced seven capsules, containing on an average twelve seeds; eight other flowers were fertilised with pollen from a distinct plant of the Brazilian stock, and produced eight capsules, containing on an average about eighty seeds: this gives a ratio of 15 seeds for the self-fertilised capsules to 100 for the crossed capsules. Later in the season twelve other flowers on these two plants were artificially self-fertilised; but they yielded only two capsules, containing three and six seeds. It appears therefore that a lower temperature than that of Brazil favours the self-fertility of this plant, whilst a still lower temperature lessens it. As soon as the two plants which had been covered by the net were uncovered, they were visited by many bees,and it was interesting to observe how quickly they became, even the more sterile plant of the two, covered with young capsules. On the following year eight flowers on plants of the Brazilian stock of self-fertilised parentage (i.e., grandchildren of the plants which grew in Brazil) were again self-fertilised, and produced five capsules, containing on an average 27.4 seeds, with a maximum in one of forty-two seeds; so that their self-fertility had evidently increased greatly by being reared for two generations in England. On the whole we may conclude that plants of the Brazilian stock are much more self-fertile in this country than in Brazil, and less so than plants of the English stock in England; so that the plants of Brazilian parentage retained by inheritance some of their former sexual constitution. Conversely, seeds from English plants sent by me to Fritz Muller and grown in Brazil, were much more self-fertile than his plants which had been cultivated there for several generations; but he informs me that one of the plants of English parentage which did not flower the first year, and was thus exposed for two seasons to the climate of Brazil, proved quite self-sterile, like a Brazilian plant, showing how quickly the climate had acted on its sexual constitution.
Abutilon darwinii.
Seeds of this plant were sent me by Fritz Muller, who found it, as well as some other species of the same genus, quite sterile in its native home of South Brazil, unless fertilised with pollen from a distinct plant, either artificially or naturally by humming-birds. (9/9. ‘Jenaische Zeitschr. fur Naturwiss’ B. 7 1872 page 22 and 1873 page 441.) Several plants were raised from these seeds and kept in the hothouse. They produced flowers very early in the spring, and twenty of them were fertilised, some with pollen from the same flower, and some with pollen from other flowers on the same plants; but not a single capsule was thus produced, yet the stigmas twenty-seven hours after the application of the pollen were penetrated by the pollen-tubes. At the same time nineteen flowers were crossed with pollen from a distinct plant, and these produced thirteen capsules, all abounding with fine seeds. A greater number of capsules would have been produced by the cross, had not some of the nineteen flowers been on a plant which was afterwards proved to be from some unknown cause completely sterile with pollen of any kind. Thus far these plants behaved exactly like those in Brazil; but later in the season, in the latter part of May and in June, they began to produce under a net a few spontaneously self-fertilised capsules. As soon as this occurred, sixteen flowers were fertilised with their own pollen, and these produced five capsules, containing on an average 3.4 seeds. At the same time I selected by chance four capsules from the uncovered plants growing close by, the flowers of which I had seen visited by humble-bees, and these contained on an average 21.5 seeds; so that the seeds in the naturally intercrossed capsules to those in the self-fertilised capsules were as 100 to 16. The interesting point in this case is that these plants, which were unnaturally treated by being grown in pots in a hothouse, under another hemisphere, with a complete reversal of the seasons, were thus rendered slightly self-fertile, whereas they seem always to be completely self-sterile in their native home.
Senecio cruentus (greenhouse varieties, commonly called Cinerarias, probably derived from several fruticose or herbaceous species much intercrossed (9/10. I am much obliged to Mr. Moore and to Mr. Thiselton Dyer for giving me information with respect to the varieties on which I experimented. Mr. Moore believes that Senecio cruentas, tussilaginis, and perhaps heritieri, maderensis and populifolius have all been more or less blended together in our Cinerarias.))
Two purple-flowered varieties were placed under a net in the greenhouse, and four corymbs on each were repeatedly brushed with flowers from the other plant, so that their stigmas were well covered with each other’s pollen. Two of the eight corymbs thus treated produced very few seeds, but the other six produced on an average 41.3 seeds per corymb, and these germinated well. The stigmas on four other corymbs on both plants were well smeared with pollen from the flowers on their own corymbs; these eight corymbs produced altogether ten extremely poor seeds, which proved incapable of germinating. I examined many flowers on both plants, and found the stigmas spontaneously covered with pollen; but they produced not a single seed. These plants were afterwards left uncovered in the same house where many other Cinerarias were in flower; and the flowers were frequently visited by bees. They then produced plenty of seed, but one of the two plants less than the other, as this species shows some tendency to be dioecious.
The trial was repeated on another variety with white petals tipped with red. Many stigmas on two corymbs were covered with pollen from the foregoing purple variety, and these produced eleven and twenty-two seeds, which germinated well. A large number of the stigmas on several of the other corymbs were repeatedly smeared with pollen from their own corymb; but they yielded only five very poor seeds, which were incapable of germination. Therefore the above three plants belonging to two varieties, though growing vigorously and fertile with pollen from either of the other two plants, were utterly sterile with pollen from other flowers on the same plant.
Reseda odorata.