In order to avoid misapprehension, I beg leave to repeat that throughout this volume a crossed plant, seedling, or seed, means one of crossed parentage, that is, one derived from a flower fertilised with pollen from a distinct plant of the same species. And that a self-fertilised plant, seedling, or seed, means one of self-fertilised parentage, that is, one derived from a flower fertilised with pollen from the same flower, or sometimes, when thus stated, from another flower on the same plant.
Ipomoea purpurea, comparison of the height and fertility of the crossed and self-fertilised plants during ten successive generations. Greater constitutional vigour of the crossed plants. The effects on the offspring of crossing different flowers on the same plant, instead of crossing distinct individuals. The effects of a cross with a fresh stock. The descendants of the self-fertilised plant named Hero. Summary on the growth, vigour, and fertility of the successive crossed and self-fertilised generations. Small amount of pollen in the anthers of the self-fertilised plants of the later generations, and the sterility of their first-produced flowers. Uniform colour of the flowers produced by the self-fertilised plants. The advantage from a cross between two distinct plants depends on their differing in constitution.
A plant of Ipomoea purpurea, or as it is often called in England the convolvulus major, a native of South America, grew in my greenhouse. Ten flowers on this plant were fertilised with pollen from the same flower; and ten other flowers on the same plant were crossed with pollen from a distinct plant. The fertilisation of the flowers with their own pollen was superfluous, as this convolvulus is highly self-fertile; but I acted in this manner to make the experiments correspond in all respects. Whilst the flowers are young the stigma projects beyond the anthers; and it might have been thought that it could not be fertilised without the aid of humble-bees, which often visit the flowers; but as the flower grows older the stamens increase in length, and their anthers brush against the stigma, which thus receives some pollen. The number of seeds produced by the crossed and self-fertilised flowers differed very little.
[Crossed and self-fertilised seeds obtained in the above manner were allowed to germinate on damp sand, and as often as pairs germinated at the same time they were planted in the manner described in the Introduction (Chapter 1), on the opposite sides of two pots. Five pairs were thus planted; and all the remaining seeds, whether or not in a state of germination, were planted on the opposite sides of a third pot, so that the young plants on both sides were here greatly crowded and exposed to very severe competition. Rods of iron or wood of equal diameter were given to all the plants to twine up; and as soon as one of each pair reached the summit both were measured. A single rod was placed on each side of the crowded pot, Number 3, and only the tallest plant on each side was measured.