The Different Forms of Flowers on Plants of the Same Species - Charles Robert Darwin (ebook reader screen TXT) 📗

though in a high degree sterile, can be fertilised by either parent-species, thus giving rise to still finer gradational links, then the presence of such linking forms in a state of nature ceases to be an argument of any weight in favour of the cowslip and primrose being varieties, and becomes, in fact, an argument on the other side. The hybrid origin of a plant in a state of nature can be recognised by four tests: first, by its occurrence only where both presumed parent-species exist or have recently existed; and this holds good, as far as I can discover, with the oxlip; but the P. elatior of Jacq., which, as we shall presently see, constitutes a distinct species, must not be confounded with the common oxlip. Secondly, by the supposed hybrid plant being nearly intermediate in character between the two parent-species, and especially by its resembling hybrids artificially made between the same two species. Now the oxlip is intermediate in character, and resembles in every respect, except in the colour of the corolla, hybrids artificially produced between the primrose and the polyanthus, which latter is a variety of the cowslip. Thirdly, by the supposed hybrids being more or less sterile when crossed inter se: but to try this fairly two distinct plants of the same parentage, and not two flowers on the same plant, should be crossed; for many pure species are more or less sterile with pollen from the same individual plant; and in the case of hybrids from heterostyled species the opposite forms should be crossed. Fourthly and lastly, by the supposed hybrids being much more fertile when crossed with either pure parent-species than when crossed inter se, but still not as fully fertile as the parent-species.
For the sake of ascertaining the two latter points, I transplanted a group of wild oxlips into my garden. They consisted of one long-styled and three short- styled plants, which, except in the corolla of one being slightly larger, resembled each other closely. The trials which were made, and the results obtained, are shown in tables 2.14, 2.15, 2.16, 2.17 and 2.18. No less than twenty different crosses are necessary in order to ascertain fully the fertility of hybrid heterostyled plants, both inter se and with their two parent-species. In this instance 256 flowers were crossed in the course of four seasons. I may mention, as a mere curiosity, that if any one were to raise hybrids between two trimorphic heterostyled species, he would have to make 90 distinct unions in order to ascertain their fertility in all ways; and as he would have to try at least 10 flowers in each case, he would be compelled to fertilise 900 flowers and count their seeds. This would probably exhaust the patience of the most patient man.
TABLE 2.14. Crosses inter se between the two forms of the common Oxlip.
Column 1: Illegitimate union. Short-styled oxlip, by pollen of short-styled oxlip: 20 flowers fertilised, did not produce one capsule.
Column 2: Legitimate union. Short-styled oxlip, by pollen of long-styled oxlip: 10 flowers fertilised, did not produce one capsule.
Column 3: Illegitimate union. Long-styled oxlip, by its own pollen: 24 flowers fertilised, produced five capsules, containing 6, 10, 20, 8, and 14 seeds. Average 11.6.
Column 4: Legitimate union. Long-styled oxlip, by pollen of short-styled oxlip: 10 flowers fertilised, did not produce one capsule.
TABLE 2.15. Both forms of the Oxlip crossed with Pollen of both forms of the Cowslip, P. veris.
Column 1: Illegitimate union. Short-styled oxlip, by pollen of short-styled cowslip: 18 flowers fertilised, did not produce one capsule.
Column 2: Legitimate union. Short-styled oxlip, by pollen of long-styled cowslip: 18 flowers fertilised, produced three capsules, containing 7, 3, and 3 wretched seeds, apparently incapable of germination.
Column 3: Illegitimate union. Long-styled oxlip, by pollen of long-styled cowslip: 11 flowers fertilised, produced one capsule, containing 13 wretched seeds.
Column 4: Legitimate union. Long-styled oxlip, by pollen of short-styled cowslip: 5 flowers fertilised, produced two capsules, containing 21 and 28 very fine seeds.
TABLE 2.16. Both forms of the Oxlip crossed with Pollen of both forms of the Primrose, P. vulgaris.
Column 1: Illegitimate union. Short-styled oxlip, by pollen of short-styled primrose: 34 flowers fertilised, produced two capsules, containing 5 and 12 seeds.
Column 2: Legitimate union. Short-styled oxlip, by pollen of long-styled primrose: 26 flowers fertilised, produced six capsules, containing 16, 20, 5, 10, 19, and 24 seeds. Average 15.7. Many of the seeds very poor, some good.
Column 3: Illegitimate union. Long-styled oxlip, by pollen of long-styled primrose: 11 flowers fertilised, produced four capsules, containing 10, 7, 5, and 6 wretched seeds. Average 7.0.
Column 4: Legitimate union. Long-styled oxlip, by pollen of short-styled primrose: 5 flowers fertilised, produced five capsules, containing 26, 32, 23, 28, and 34 seeds. Average 28.6.
TABLE 2.17. Both forms of the Cowslip crossed with Pollen of both forms of the Oxlip.
Column 1: Illegitimate union. Short-styled cowslip, by pollen of short-styled oxlip: 8 flowers fertilised, did not produce one capsule.
Column 2: Legitimate union. Long-styled cowslip, by pollen of short-styled oxlip: 8 flowers fertilised, produced one capsule, containing 26 seeds.
Column 3: Illegitimate union. Long-styled cowslip, by pollen of long-styled oxlip: 8 flowers fertilised, produced three capsules, containing 5, 6 and 14 seeds. Average 8.3.
Column 4: Legitimate union. Short-styled cowslip, by pollen of long-styled oxlip: 8 flowers fertilised, produced 8 capsules, containing 58, 38, 31, 44, 23, 26, 37, and 66 seeds. Average 40.4.
TABLE 2.18. Both forms of the Primrose crossed with Pollen of both forms of the Oxlip.
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,