Column 1: Illegitimate union. Short-styled primrose, by pollen of short-styled oxlip: 8 flowers fertilised, did not produce one capsule.
Column 2: Legitimate union. Long-styled primrose, by pollen of short-styled oxlip: 8 flowers fertilised, produced two capsules, containing 5 and 2 seeds.
Column 3: Illegitimate union. Long-styled primrose, by pollen of long-styled oxlip: 8 flowers fertilised, produced 8 capsules, containing 15, 7, 12, 20, 22, 7, 16, and 13 seeds. Average 14.0.
Column 4: Legitimate union. Short-styled primrose, by pollen of long-styled oxlip: 8 flowers fertilised, produced 4 capsules, containing 52, 52, 42, and 49 seeds, some good and some bad. Average 48.7.
We see in Tables 2/14 to 2/18 the number of capsules and of seeds produced, by crossing both forms of the oxlip in a legitimate and illegitimate manner with one another, and with the two forms of the primrose and cowslip. I may premise that the pollen of two of the short-styled oxlips consisted of nothing but minute aborted whitish cells; but in the third short-styled plant about one- fifth of the grains appeared in a sound condition. Hence it is not surprising that neither the short-styled nor the long-styled oxlip produced a single seed when fertilised with this pollen. Nor did the pure cowslips or primroses when illegitimately fertilised with it; but when thus legitimately fertilised they yielded a few good seeds. The female organs of the short-styled oxlips, though greatly deteriorated in power, were in a rather better condition than the male organs; for though the short-styled oxlips yielded no seed when fertilised by the long-styled oxlips, and hardly any when illegitimately fertilised by pure cowslips or primroses, yet when legitimately fertilised by these latter species, especially by the long-styled primrose, they yielded a moderate supply of good seed.
The long-styled oxlip was more fertile than the three short-styled oxlips, and about half its pollen-grains appeared sound. It bore no seed when legitimately fertilised by the short-styled oxlips; but this no doubt was due to the badness of the pollen of the latter; for when illegitimately fertilised (Table 2.14) by its own pollen it produced some good seeds, though much fewer than self- fertilised cowslips or primroses would have produced. The long-styled oxlip likewise yielded a very low average of seed, as may be seen in the third compartment of Tables 2.15 to 2.18, when illegitimately fertilised by, and when illegitimately fertilising, pure cowslips and primroses. The four corresponding legitimate unions, however, were moderately fertile, and one (namely that between a short-styled cowslip and the long-styled oxlip in Table 2.17) was nearly as fertile as if both parents had been pure. A short-styled primrose legitimately fertilised by the long-styled oxlip (Table 2.18) also yielded a moderately good average, namely 48.7 seeds; but if this short-styled primrose had been fertilised by a long-styled primrose it would have yielded an average of 65 seeds. If we take the ten legitimate unions together, and the ten illegitimate unions together, we shall find that 29 per cent of the flowers fertilised in a legitimate manner yielded capsules, these containing on an average 27.4 good and bad seeds; whilst only 15 per cent of the flowers fertilised in an illegitimate manner yielded capsules, these containing on an average only 11.0 good and bad seeds.
In a previous part of this chapter it was shown that illegitimate crosses between the long-styled form of the primrose and the long-styled cowslip, and between the short-styled primrose and short-styled cowslip, are more sterile than legitimate crosses between these two species; and we now see that the same rule holds good almost invariably with their hybrid offspring, whether these are crossed inter se, or with either parent-species; so that in this particular case, but not as we shall presently see in other cases, the same rule prevails with the pure unions between the two forms of the same heterostyled species, with crosses between two distinct heterostyled species, and with their hybrid offspring.
Seeds from the long-styled oxlip fertilised by its own pollen were sown, and three long-styled plants raised. The first of these was identical in every character with its parent. The second bore rather smaller flowers, of a paler colour, almost like those of the primrose; the scapes were at first single- flowered, but later in the season a tall thick scape, bearing many flowers, like that of the parent oxlip, was thrown up. The third plant likewise produced at first only single-flowered scapes, with the flowers rather small and of a darker yellow; but it perished early. The second plant also died in September; and the first plant, though all three grew under very favourable conditions, looked very sickly. Hence we may infer that seedlings from self-fertilised oxlips would hardly be able to exist in a state of nature. I was surprised to find that all the pollen-grains in the first of these seedling oxlips appeared sound; and in the second only a moderate number were bad. These two plants, however, had not the power of producing a proper number of seeds; for though left uncovered and surrounded by pure primroses and cowslips, the capsules were estimated to include an average of only from fifteen to twenty seeds.
From having many experiments in hand, I did not sow the seed obtained by crossing both forms of the primrose and cowslip with both forms of the oxlip, which I now regret; but I ascertained an interesting point, namely, the character of the offspring from oxlips growing in a state of nature near both primroses and cowslips. The oxlips were the same plants which, after their seeds had been collected, were transplanted and experimented on. From the seeds thus obtained eight plants were raised, which, when they flowered, might have been mistaken for pure primroses; but on close comparison the eye in the centre of the corolla was seen to be of a darker yellow, and the peduncles more elongated. As the season advanced, one of these plants threw up two naked scapes, 7 inches in height, which bore umbels of flowers of the same character as before. This fact led me to examine the other plants after they had flowered and were dug up; and I found that the flower-peduncles of all sprung from an extremely short common scape, of which no trace can be found in the pure primrose. Hence these plants are beautifully intermediate between the oxlip and the primrose, inclining rather towards the latter; and we may safely conclude that the parent oxlips had been fertilised by the surrounding primroses.
From the various facts now given, there can be no doubt that the common oxlip is a hybrid between the cowslip (P. veris, Brit. Fl.) and the primrose (P. vulgaris, Brit. Fl.), as has been surmised by several botanists. It is probable that oxlips may be produced either from the cowslip or the primrose as the seed- bearer, but oftenest from the latter, as I judge from the nature of the stations in which oxlips are generally found (2/13. See also on this head Hardwicke’s ‘Science Gossip’ 1867 pages 114, 137.), and from the primrose when crossed by the cowslip being more fertile than, conversely, the cowslip by the primrose. The hybrids themselves are also rather more fertile when crossed with the primrose than with the cowslip. Whichever may be the seed-bearing plant, the cross is probably between different forms of the two species; for we have seen that legitimate hybrid unions are more fertile than illegitimate hybrid unions. Moreover a friend in Surrey found that 29 oxlips which grew in the neighbourhood of his house consisted of 13 long-styled and 16 short-styled plants; now, if the parent-plants had been illegitimately united, either the long- or short-styled form would have greatly preponderated, as we shall hereafter see good reason to believe. The case of the oxlip is interesting; for hardly any other instance is known of a hybrid spontaneously arising in such large numbers over so wide an extent of country. The common oxlip (not the P. elatior of Jacq.) is found almost everywhere throughout England, where both cowslips and primroses grow. In some districts, as I have seen near Hartfield in Sussex and in parts of Surrey, specimens may be found on the borders of almost every field and small wood. In other districts the oxlip is comparatively rare: near my own residence I have found, during the last twenty-five years, not more than five or six plants or groups of plants. It is difficult to conjecture what is the cause of this difference in their number. It is almost necessary that a plant, or several plants belonging to the same form, of one parent-species, should grow near the opposite form of the other parent-species; and it is further necessary that both species should be frequented by the same kind of insect, no doubt a moth. The cause of the rare appearance of the oxlip in certain districts may be the rarity of some moth, which in other districts habitually visits both the primrose and cowslip.