(5/8. 'Bestimmung des Werthes von Species und Varietat' 1869 pages 47-72.) But the above distance would be very far from sufficient to prevent intercrossing: cabbages have been known to cross at several times this distance; and the careful Gartner gives many instances of plants growing at from 600 to 800 yards apart fertilising one another. (5/9. 'Kenntnis der Befruchtung' 1844 pages 573, 577.) Professor Hoffman even maintains that the flowers of the kidney-bean are specially adapted for self-fertilisation. He enclosed several flowers in bags; and as the buds often dropped off, he attributes the partial sterility of these flowers to the injurious effects of the bags, and not to the exclusion of insects. But the only safe method of experimenting is to cover up a whole plant, which then never suffers.
Self-fertilised seeds were obtained by moving up and down in the same manner as bees do the wing-petals of flowers protected by a net; and crossed seeds were obtained by crossing two of the plants under the same net. The seeds after germinating on sand were planted on the opposite sides of two large pots, and equal-sized sticks were given them to twine up. When 8 inches in height, the plants on the two sides were equal. The crossed plants flowered before the self-fertilised in both pots. As soon as one of each pair had grown to the summit of its stick both were measured.
TABLE 5/53. Phaseolus multiflorus.
Column 1: Number (Name) of Pot.
Column 2: Crossed Plants.
Column 3: Self-fertilised Plants.
Pot 1 : 87 : 84 6/8. Pot 1 : 88 : 87. Pot 1 : 82 4/8 : 76.
Pot 2 : 90 : 76 4/8. Pot 2 : 82 4/8 : 87 4/8.
Total : 430.00 : 411.75.
The average height of the five crossed plants is 86 inches, and that of the five self-fertilised plants 82.35; or as 100 to 96. The pots were kept in the greenhouse, and there was little or no difference in the fertility of the two lots. Therefore as far as these few observations serve, the advantage gained by a cross is very small.
Phaseolus vulgaris.
With respect to this species, I merely ascertained that the flowers were highly fertile when insects were excluded, as indeed must be the case, for the plants are often forced during the winter when no insects are present. Some plants of two varieties (namely Canterbury and Fulmer's Forcing Bean) were covered with a net, and they seemed to produce as many pods, containing as many beans, as some uncovered plants growing alongside; but neither the pods nor the beans were actually counted. This difference in self-fertility between Phaseolus vulgaris and multifloris is remarkable, as these two species are so closely related that Linnaeus thought that they formed one. When the varieties of Phaseolus vulgaris grow near one another in the open ground, they sometimes cross largely, notwithstanding their capacity for self-fertilisation. Mr. Coe has given me a remarkable instance of this fact with respect to the negro and a white-seeded and a brown-seeded variety, which were all grown together. The diversity of character in the seedlings of the second generation raised by me from his plants was wonderful. I could add other analogous cases, and the fact is well-known to gardeners. (5/10. I have given Mr. Coe's case in the 'Gardeners' Chronicle' 1858 page 829. See also for another case ibid page 845.)
Lathyrus odoratus.
Almost everyone who has studied the structure of papilionaceous flowers has been convinced that they are specially adapted for cross-fertilisation, although many of the species are likewise capable of self-fertilisation. The case therefore of Lathyrus odoratus or the sweet-pea is curious, for in this country it seems invariably to fertilise itself. I conclude that this is so, as five varieties, differing greatly in the colour of their flowers but in no other respect, are commonly sold and come true; yet on inquiry from two great raisers of seed for sale, I find that they take no precautions to insure purity--the five varieties being habitually grown close together.