NB.--In the above cases, excepting in that of Eschscholtzia, the plants derived from a cross with a fresh stock belong on the mother-side to the same stock with the intercrossed and self-fertilised plants, and to the corresponding generation.
These cases show us how greatly superior in innate fertility the seedlings from plants self-fertilised or intercrossed for several generations and then crossed by a fresh stock are, in comparison with the seedlings from plants of the old stock, either intercrossed or self-fertilised for the same number of generations. The three lots of plants in each case were left freely exposed to the visits of insects, and their flowers without doubt were cross-fertilised by them.
Table 9/E further shows us that in all four cases the intercrossed plants of the same stock still have a decided though small advantage in fertility over the self-fertilised plants.
With respect to the state of the reproductive organs in the self-fertilised plants of Tables 9/D and 9/E, only a few observations were made. In the seventh and eighth generation of Ipomoea, the anthers in the flowers of the self-fertilised plants were plainly smaller than those in the flowers of the intercrossed plants. The tendency to sterility in these same plants was also shown by the first-formed flowers, after they had been carefully fertilised, often dropping off, in the same manner as frequently occurs with hybrids. The flowers likewise tended to be monstrous. In the fourth generation of Petunia, the pollen produced by the self-fertilised and intercrossed plants was compared, and they were far more empty and shrivelled grains in the former.
RELATIVE FERTILITY OF FLOWERS CROSSED WITH POLLEN FROM A DISTINCT PLANT AND WITH THEIR OWN POLLEN. THIS HEADING INCLUDES FLOWERS ON THE PARENT-PLANTS, AND ON THE CROSSED AND SELF-FERTILISED SEEDLINGS OF THE FIRST OR A SUCCEEDING GENERATION.
I will first treat of the parent-plants, which were raised from seeds purchased from nursery-gardens, or taken from plants growing in my garden, or growing wild, and surrounded in every case by many individuals of the same species. Plants thus circumstanced will commonly have been intercrossed by insects; so that the seedlings which were first experimented on will generally have been the product of a cross. Consequently any difference in the fertility of their flowers, when crossed and self-fertilised, will have been caused by the nature of the pollen employed; that is, whether it was taken from a distinct plant or from the same flower. The degrees of fertility shown in Table 9/F, were determined in each case by the average number of seeds per capsule, ascertained either by counting or weighing.
Another element ought properly to have been taken into account, namely, the proportion of flowers which yielded capsules when they were crossed and self-fertilised; and as crossed flowers generally produce a larger proportion of capsules, their superiority in fertility, if this element had been taken into account, would have been much more strongly marked than appears in Table 9/F. But had I thus acted, there would have been greater liability to error, as pollen applied to the stigma at the wrong time fails to produce any effect, independently of its greater or less potency. A good illustration of the great difference in the results which sometimes follows, if the number of capsules produced relatively to the number of flowers fertilised be included in the calculation, was afforded by Nolana prostrata. Thirty flowers on some plants of this species were crossed and produced twenty-seven capsules, each containing five seeds; thirty-two flowers on the same plants were self-fertilised and produced only six capsules, each containing five seeds. As the number of seeds per capsule is here the same, the fertility of the crossed and self-fertilised flowers is given in Table 9/F as equal, or as 100 to 100. But if the flowers which failed to produce capsules be included, the crossed flowers yielded on an average 4.50 seeds, whilst the self-fertilised flowers yielded only 0.94 seeds, so that their relative fertility would have been as 100 to 21.